Cyclostomata

“ The Cyclostomata and the Phylactolaemata have the most modified forms of larva. That of the former group may owe some of its peculiarities to the occurrence of a remarkable process of embryonic fission, which takes place in the ovicell, and as the result of which each egg gives rise to a large number of larvae. [579] The Phylactolaemata have a larva which is not unlike that of Bowerbankia.
Fig. 253.—A, Aboral view of free larva of Lepralia foliacea Ell. and Sol.; a, long cilia of pyriform organ; g, aboral groove: B, longitudinal section of embryo of Alcyonidium, × 135; c, ciliated ring ”

“ Cyclostomata. [523] —Body-wall densely calcareous, the zooecia being more or less tubular, usually with a circular orifice (Fig. 237) .
2. Cheilostomata. [524] —Body-wall of varying consistency. The orifice is closed, in the retracted state of the polypide, by a chitinous lip or "operculum," which is more or less semicircular (Figs. 239, 241) .
3. Ctenostomata. [525] —Body-wall always soft. The cavity into which the tentacles are retracted is closed by a frill-like membrane, the edges of whose folds have some resemblance to the teeth of a comb. ”

“ In the Cyclostomata a reduction of the number of gill slits takes place, and an increased area for respiration is provided by the gill pouches lined by pleated folds of entodermal mucous membrane; these form the simplest type of true internal gills. In the larval lamprey (Ammocoetes) there are eight gill slits opening from the pharynx, but in the adult (Petromyzon) they are reduced to seven, and a septum grows forward separating the ventral or bronchial part of the pharynx from the dorsal or digestive part. ”

“ The alimentary canal, which may be represented by a vestigial structure, is accordingly not functional, and the larva does not become pelagic. A pyriform organ is present in most Gymnolaemata as well as the sucker by which fixation is effected. As in the case of Cyphonautes, the larval organs degenerate and the larva becomes the ancestrula from which a polypide is developed as a bud. In the Cyclostomata the primary embryo undergoes repeated fission without developing definite organs, and each of the numerous pieces so formed becomes a free larva, which possesses no alimentary canal. ”

“ In certain genera, however, and particularly in some Ctenostomes (Fig. 238) , and in most of the Entoprocta, the {480}individuals grow out at intervals from a cylindrical stem or "stolon" (st) , which is not composed of zooecia.
The Cyclostomata may assume an erect or encrusting habit. Their zooecia are always more or less cylindrical; the upper ends being often completely free, although in many cases the whole zooecium is closely adnate to its neighbours. In the breeding season the forms which belong to this group are provided with curious "ovicells," which contain the embryos. ”

“ We owe our first accurate knowledge of the Monorrhina, or Cyclostoma, to the great zoologist, Johannes Müller of Berlin; his classical work on the “Comparative Anatomy of the Myxinoida” forms the foundation of our modern views on the structure of the Vertebrate animals. He distinguished two distinct groups among the Cyclostoma, which we shall consider as sub-classes.
The first sub-class consists of the Hags (Hyperotreta, or Myxinoida) . They live in the sea as parasites upon other fish, into whose skin they penetrate (Myxine, Bdellostoma) . ”

“ Hence they are obviously the last survivors of a very ancient class of Vertebrates, that was far from attaining the advanced organisation of the true fish. To mention only the chief points, the Cyclostoma show no trace of pairs of limbs. Their mucous skin is quite naked and smooth and devoid of scales. There is no bony skeleton. A very rudimentary skull is developed at the foremost end of their chorda. At this point a soft membranous (partly turning into cartilage) , small skull-capsule is formed, and encloses the brain. ”

“ The Dipnoi have overlapping cycloid scales. The rounded scales of Amia and of many fossil ganoids such as Holoptychius are shaped like cycloid scales, but differ from them in being more or less coated with enamel. In Eels and some other Teleosteans the scales are completely degenerate and have almost disappeared. Some Teleosteans, like Diodon hystrix, have scales with triradiate roots from which arise long sharp spines directed backwards. These scales, which resemble teeth, contain no enamel; they become erect when the fish inflates its body into a globular form. ”

“ In a species of Platygaster which is parasitic in the larva of Cecidomia, the first larva (Cyclops stage) is of a remarkable shape, not like an insect, but rudely resembling a parasitic Copepod crustacean. In this condition it clings to the inside of its host by means of its hook-like jaws, moving about, as Ganin says, like a Cestodes embryo with its well-known six hooks. In this stage it has no nervous, vascular, or respiratory system, and the digestive canal is a blind one (Fig. 651) . ”

“ In some notes forwarded to 'Loudon's Magazine' in 1831, Guilding mentions a "Cyclostoma" common in the Virgin Islands, which, having given out a mucous thread, closes the operculum, and swings by the thread when hardened by the air, safe from ants and other enemies. ”

“ A genus of microscopic Foraminifera.
VALVATA. Müll. Fam. Peristomata, Lam. Cricostomata, Bl.—Descr. Thin, turbinated; spire short, composed of from three to six rounded whorls; aperture circular; peritreme acute, entire; operculum horny, spiral.—Obs. This genus of small shells resembles Cyclostoma, from which the recent species may be known by the horny texture of the external surface, being fresh-water shells. The fossils of course belong to the fresh-water formations. V. piscinalis, fig. 322. Europe and North America. ”

“ Very general assent could be obtained for the proposition that one stage in the ancestry of the Vertebrates must have been not unlike a simplified Cyclostome, a bilaterally symmetrical coelomic animal, elongated and fish-like in shape, but without paired limbs, with a smooth, soft skin, a ventral mouth without teeth or lower jaw and probably surrounded by labial palps, with lateral gill-slits and a ventro-posterior anus; with an unsegmented notochord and a dorsal tubular nerve cord. ”

“ They usually cover the body more or less evenly and are imbricated like shingles on a roof, the free edge being turned backward. Such normal scales are of two types, ctenoid or cycloid. Ctenoid scales have a comb-edge of fine prickles or cilia; cycloid scales have the edges smooth. These two types are not very different, and the one readily passes into the other, both being sometimes seen on different parts of the same fish. In general, however, the more primitive representatives of the typical fishes, those with abdominal ventrals and without spines in the fins, have cycloid or smooth scales. ”

“ The Cyclostomes.—The next group of living fishes, the Cyclostomes, including the hagfishes and lampreys,—fishes with small skull and brain but without limbs or jaws,—stands at a great distance above the lancelet in complexity of structure, and equally far from the true fishes in its primitive simplicity. In fact the lamprey is farther from the true fish in structure than a perch is from an eagle. ”

“ The float of the pelagic Janthina, to which the egg-capsules are attached, probably is also formed by the secretion of the pedal gland.
Other glands opening on or near the foot are: (1) The suprapedal gland opening in the middle line between the snout and the anterior border of the foot. It is most commonly found in sessile forms and in terrestrial genera such as Cyclostoma; (2) the anterior pedal gland opening into the anterior groove of the foot, generally present in aquatic species; (3) dorsal posterior mucous glands in certain Cyclostomatidae. ”

“ Behind the eye there is a row of seven gill-clefts visible on the left, in front the round suctorial mouth.
We may divide the Craniota generally into Cyclostoma (“round-mouthed”) and Gnathostoma (“jaw-mouthed”) . There are only a few groups of the former in existence now, but they are very interesting, because in their whole structure they stand midway between the Acrania and the Gnathostoma. They are much more advanced than the Acrania, much less so than the fishes, and thus form a very welcome connecting-link between the two groups. ”

“ Nothing can give one so good a notion of the plasticity of organic forms as such a method. The endless varieties and intermediate links which exist amongst dogs is the nearest example to it with which ordinary observers are familiar.
But the cyclostoma is a snail which introduces one to still deeper questions. It belongs in all our scientific classifications to the group of lung-breathing mollusks, like the common garden snail. Yet it has one remarkable peculiarity: it possesses an operculum, or door to its shell, like that of the periwinkle. ”

“ Fig. 279.—Transverse section of female Guinea worm; u., uterus containing embryos; i., intestinal canal; o., ovary. (After Leuckart.)
The larvæ enter the Cyclops, according to most authorities, by penetrating the exoskeleton, but according to Leiper this is impossible; they must enter by the mouth and penetrate the gut in order to reach the body cavity. In eight days moult 1 takes place, the striated cuticle being cast off. In ten days moult 2 takes place. In five weeks the larva is mature. If now the infected Cyclops is placed in 0·2 per cent. ”

“ The entire Achatina group is peculiar, and takes, especially in W. Africa, some curious forms (Columna, Perideris, Pseudachatina) . Carnivorous Mollusca (Ennea, Gibbus, etc.) are highly developed, especially in the south and east, the largest known helicoid form (Aerope) being from Natal. In the possession of these types of the Agnatha, Africa is more closely related to the Australasian than to the Oriental region. The true Cyclostoma are entirely peculiar to the region, but are absent from West Africa. ”

“ Most of the species are of some economic value as food Fishes. Psammobatis, with a circular disc, frequents the southern coasts of South America, and Platyrhina the coasts of India, China, and Japan.
The family ranges from the Upper Cretaceous, in which, as well as in different Tertiary deposits, it is represented by species of Raia. An extinct genus, Cyclobatis, with a circular or oval disc, occurs in the Upper Cretaceous of Mount Lebanon. ”

“ Smerdis minutus. Lign. 203.—A pretty ctenoidian fish, from one to three inches long, about the dimensions of a perch a year old, is very common in the marls of Aix in Provence, Wond. p. 260; and many are often found grouped together in every variety of position. This species is characterised by the elevated anterior rays of the dorsal, and the wide and very forked caudal fin.
Several ctenoidian and cycloidian fishes have been found in the north of the Brazils, by Mr. Gardiner, in strata probably of the Cretaceous epoch.
Fossil Cycloid Fishes. (Poiss. Foss. tom. v.) ”

“ The shells of the genus Cyclas are oval, transverse, equivalved bivalves, with the hinge-teeth very small: the substance of the shell is thin and fragile; the figures in Wond. and Ly. accurately represent the appearance of the fossil Cyclades of the Wealden, and tertiary strata. Entire layers of two or three species of these shells occur in the argillaceous deposits of the Wealden, generally in a friable state, but from among the masses of crushed shells, perfect specimens may be obtained, and sometimes with the remains of the epidermis and ligament. ”

“ The cyclostome fishes have, so far, been discovered only in Scotland, in the tiny Palaeospondylus. Amphibian remains have been found in the Devonian of Belgium; and footprints supposed to belong to a creature of the same class (Thinopus antiquus) have been described by Professor Marsh from the Chemung formation of Pennsylvania.
Plant Life.—In the lacustrine deposits of the Old Red Sandstone we find the earliest well-defined assemblage of terrestrial plants. In some regions so abundant are the vegetable remains that in places they form thin seams of veritable coal. ”

“ Cyclops, I swear by Neptune who begot thee, ⁠241
By mighty Triton and by Nereus old,
Calypso and the glaucous Ocean Nymphs,
The sacred waves and all the race of fishes—
Be these the witnesses, my dear sweet master, ⁠245
My darling little Cyclops, that I never
Gave any of your stores to these false strangers;—
If I speak false may those whom most I love,
My children, perish wretchedly!
⁠Chorus. ⁠There stop!
I saw him giving these things to the strangers. ⁠250
If I speak false, then may my father perish,
But do not thou wrong hospitality.
⁠Cyclops. You lie! ”

“ The species are imperfectly known. The defect of mouth-organs probably does not apply to the period of youth, which some of them spend parasitically in the body-cavity of worms (Giard, 1896) . To the Cyclopidae six genera are allotted by Giesbrecht in 1900. Cyclops (O. F. Müller, 1776) , though greatly restricted since Müller’s time, still has several scores of species abundantly peopling inland waters of every kind and situation, without one that can be relied on as exclusively marine like the species of Oithona (Baird) . ”

“ They either occur in long chains or are stipitate or sessile. If it is further assumed that the fresh-water diatoms are found in greater abundance in later periods, the action of running streams makes necessary the provision of some means by which the species may continue to colonize. This may be recognized in the occurrence of linear forms chiefly in streams. Circular forms, such as Cyclotella which have no raphe, are found in quiet waters, such as pools or ditches, and never exist living in running streams. ”

“ After the outer layer of shell is removed, it presents a delicate pearly appearance. Its lingual band (No. 123) closely resembles Trochus; it is long and narrow, the median teeth are broadest, with five recurved laterals, and numerous 558 rows of uncini, slender and hooked. A single row only is represented in the plate.
Cyclotus translucidus, a family of operculate land-shells, belongs to the Cyclostomatidæ. The teeth shown in No. 110, formula 3 · 1 · 3, are arranged in slightly divergent rows on a narrow band; they are more or less subquadrate, recurved, with their central cusps prolonged. ”