Annelida

“ Originally the body is composed of a congeries of cells, segment after segment being added, exactly in the same manner as in the case of the Annelida, with which in structure they closely agree, being adapted, however, to live on land. The Annelida seem then to be the ancestors of the Myriapoda, Insects, and Spiders, the Myriapoda retaining much of the Annelid structure through life, whereas the Insect is an Annelid or Myriapod only when in a larval or undeveloped condition. That the development of the different kinds of insects has been gradual. ”

“ Very definite and comparatively uniform, are the relations between shapes and circumstances among the Annulosa: including under that title the Annelida and the Arthropoda. The agreements and the disagreements are equally instructive.
At one time or other of its life, if not throughout its life, every annulose animal is locomotive; and its temporary or permanent locomotion, being carried on with one end habitually foremost and one surface habitually uppermost, it fulfils those conditions under which bilateral symmetry arises. ”

“ The wonderful process of the Terebella multiplying itself by gemmation, described in such a spirited manner in the "Sea-side Studies, (page 61,) is far too interesting not to find a place in our Work;—it is quite a new feature in its history, although it has been long known to occur in nearly allied Annelida, the tail of the Nereis, for instance, being still the tail of its offspring, and however often the body may divide, still the same tail remains attached to the hinder portion, so that this part of the animal may be said to enjoy a kind of immunity from death. ”

“ Here no one would doubt the homology of the mouth and anus throughout the group; yet within the limits of a single genus—Peripatus—they show the most diverse modes of development. In Annelids the blastopore sometimes becomes the mouth (most Chaetopoda) ; sometimes it becomes the anus (Serpula) ; sometimes it closes up, giving rise to neither, though in this case it may assume the form of a long slit along the ventral surface before disappearing. In Mollusca its fate presents the same variations as in Annelida. ”

“ THE GOLDEN-COMBED WORM.
These tube-forming Annelida are very interesting creatures; and many of them possess great beauty from the exquisitely delicate, and often highly coloured appendages with which they are furnished. Through the kindness of the Rev. C. Kingsley I possess a full grown specimen of the Golden-combed Worm (Amphitrite auricoma) . When I at first had him he was very shy and timid, but after a week or two he grew more familiar, and would protrude his gilt combs, and carry on his avocations, as if quite at home. ”

“ If we look at a Serpula recently dead,—which we may readily do, since it is the habit of most tubicolous Annelida to come out to die,—we shall find, with a lens, a pale yellow line running along the upper surface of each foot, transversely to the length of the body. ”

“ The whole effect of the Nereid gliding through the sea is so beautiful that it has called forth the admiration of the poets. The Annelida increase their length by adding segments to those already formed. In this respect they resemble the Centipedes, etc., which belong to the Myriapoda, of which we will speak again. The Rotatoria (Fig. 35) , or third division of the Articulated Worms, are microscopical. They live in fresh or salt water; they are composed of a head and a body; sometimes the head and body coalesce. ”

“ Not very much interest attaches to the developmental stage of the Annelida. They issue forth from ova, and the embryo so closely resemble ciliated polypes, that competent observers have mistaken them for animals belonging to a lower class; a few hours’ careful watching is sufficient to dispel a belief of the kind, when the embryonic, globular, or shapeless mass is seen to assume a form of segmentation, and soon the various internal organs become more and more developed, eye spots appear, and the young animal arrives at the adult stage of its existence. ”

“ The other aquatic worms which are never exposed to the air, multiplying and diversifying their races with time, and gradually making progress in the complication of their structure, have caused the formation of the Annelida, Cirripedia, and molluscs, which together form an uninterrupted portion of the animal scale. ”

“ The Chaetopoda embrace the following smaller groups or Orders:—I. Archiannelida, II. Polychaeta, III. Myzostomaria, IV. Oligochaeta. The Archiannelida, although without the characteristic chaetae, are yet anatomically so similar to the true Chaetopoda that they must be included in the group, just as certain fishes are classed as "Vertebrata," although they do not possess vertebrae. The old term Annelida is sometimes used to include the above-mentioned groups, together with the Gephyrea [292] and the Hirudinea or leeches. ”

“ The marine shell-constructing Annelida present some peculiarities of distribution worthy of notice by geologists, such as the prevalence of Serpula serrulata in the deeper regions of the northern provinces and the constant occurrence of species of Ditrupa at great depths in several parts of our seas far from land.
Our knowledge of the species and distribution of the British Mollusca is very complete, and sufficient to enable us to apply it to the elucidation of geological problems with safety and effect. ”

“ In the lower Annelida, for example, "after the number of segments in the body has been greatly multiplied by gemmation, a separation of those of the posterior portion begins to take place; a constriction forms itself about the beginning of the posterior third of the body, in front of which the alimentary canal undergoes a dilatation, whilst on the segment behind it a proboscis and eyes are developed, so as to form the head of the young animal which is to be budded off ”

“ We have yet to advert to the most interesting class of facts connected with the laws of organic development. It is only in recent times that physiologists have observed that each animal passes, in the course of its germinal history, through a series of changes resembling the permanent forms of the various orders of animals inferior to it in the scale. Thus, for instance, an insect, standing at the head of the articulated animals, is, in the larva state, a true annelid, or worm, the annelida being the lowest in the same class. ”

“ Minute Mollusca, both bivalve and univalve, uncouth-formed Crustacea, tiny Starfishes, and especially Annelida, will pretty certainly reward the investigator. The last-named Class occurs in remarkable abundance and variety; while if, after you have gone round the glass, noticing particularly the very edge of the surface-line, you pass your eye, assisted by the lens, carefully over the surfaces of the bits of stone, you will probably find many more creatures, such as tube-dwelling Annelides, the smaller Zoophytes, and several species of the delicate Bryozoa. ”

“ Dr. Williams, in his admirable 'Report on the British Annelida,' has, I think fallen into an error with regard to this species; or at least his statements in this particular do not agree with my own observation. After describing the mode in which the posterior extremity in A. alveolata is contracted into a true cylindrical tail, which, turning upwards, returns along parallel to the body, in order to project the fæcal refuse to the anterior extremity of the tube, he ascribes a similar structure to the present species. ”

“ In Cephalopods among Mollusca—the Central nervous system. development of which is remarkable from the almost complete absence of features which are supposed to have an ancestral significance—and in one or two other forms, it has been said to develop from the mesoderm; but apart from 320 these exceptional and perhaps doubtful cases, the central nervous system of all embryos arises as thickenings of the ectoderm, and in the groups above mentioned, namely, Annelida, Mollusca, Arthropoda and Vertebrata, and probably others, from the ectoderm of the blastoporal surface of the body. ”

“ The annelid had hit upon a most advantageous line of development, which led ultimately to the insect. The study of the insect will show us clearly the advantages and defects of the annelid plan. First of all, the insect, like the mollusk, has an external skeleton. But the skeleton of the mollusk was purely protective, a hindrance to locomotion. That of the insect is still somewhat protective, but is mainly, almost purely, locomotive. It is never allowed to become so heavy as to interfere with locomotion. ”

“ But this animal does not live alone in this asylum. He is not so much of an anchorite as he appears to be, for by his side an annelid usually instals himself as a messmate, which forms with the Pagurus one of the most terrible associations that are known. This annelid is a long worm, like all the nereids, whose supple and undulating body is armed along its sides with arrows, lances, pikes, and poniards, the wounds of which are always dangerous. It is a living panoply which glides furtively into the enemy’s camp without giving the alarm. ”

“ The soft bodies of certain species are protected by a coat, or tube, formed by the agglutination of sand, or other foreign substances, as in the Sabella (Lign. 123, fig. 6, p. 385) .
The fossil remains of the testaceous Annelides are very abundant in some deposits; and even the naked, flexible, soft-bodied forms have left proofs of their existence in some of the most ancient sedimentary rocks. Traces of nine species, belonging to five genera of these soft, naked Annelides, have been observed in the Silurian strata of Britain.
Lign. 166. Fossil impression of Nereis. Silurian strata. ”

“ The Annelids, in which the body consists of a long series of connected rings or segments, as in the earth-worm, are one of the highest groups of these worm-like creatures, and some branch of them developed a pair of feet (as in the caterpillar) on each segment of the body and a tough, chitinous coat. Thus arose the early Arthropods, on tough-coated, jointed, articulated animals. Some of these remained in the water, breathing by means of gills, and became the Crustacea. ”

“ The annelid had two anterior segments differing from those of the rest of the body; these may, [66] perhaps, be considered as the foreshadowings of a structure not yet realized; they can only by courtesy be called a head. Thirdly, the insect has legs. The annelid had fin-like parapodia, approaching the legs of insects about as closely as the fins of a fish approach the legs of a mammal. The reproductive and digestive systems, while somewhat improved, are not very markedly higher than those of annelids. The excretory system has more work to perform and reaches a rather higher development. ”

“ The larvæ of Annelids are very various; but amid their differences there is a recognizable type. “The Trochophore is the typical larval form of the Annelid stem”: a trochophore being a curious spheroidal ciliated structure suggestive of cœlenterate affinities. And this unsegmented larva, representing the remote ancestor from which the many Annelid types diverged, is similar to the larvæ of the Rotifera and the Mollusca: a trochophore is common to all these great classes. ”

“ Such an origin signifies that the origin of the excretory organs of the vertebrate must not be looked for in the segmental organs of the annelid, but rather in such modified annelid organs as would naturally exist in a primitive arthropod group. The nature of such organs may be inferred, owing to the fortunate circumstance that so primitive an arthropod as Peripatus still exists, and we may conclude that the protostracan ancestor possessed in every segment a pair of appendages and a pair of cœlomic cavities, which extended into the base of these appendages. ”

“ Is it not to be marvelled at, that these humble beings should see without eyes, and smell without a nose? It is not affirmed that this is literally and entirely true; but it is exact to a degree enough to prove the wondrous manner in which the sense of touch is made to supersede all the other senses.
"Whether progressing on the solid surface, or moving through water, or tunneling the sand, advancing or retreating in its tube, the Annelid performs muscular feats, distinguished at once for their complexity and harmony. In grace of coil the little Worm excels the Serpent. ”

“ But the annelids were probably the first animals to develop an eye capable of forming an image of external objects. The importance of this organ in the pursuit of food or the escape from enemies can scarcely be over-estimated. The lining of the mouth and pharynx can be protruded as a proboscis, and drawn back by powerful muscles, and is armed with two or more horny claws. Eyes and claws gave them a great advantage over their not quite blind but really visionless and comparatively defenceless neighbors, and they must have wrought terrible extinction of lower and older forms. ”

“ Stephens's genus only into the order, leaving the remainder of the species undisposed of. The development of the antlia in Acronycta discovers as near an approach to Noctua, as Lithosia does to Tinea; but the bear-like, cocoon-spinning larva place these insects in close alliance with the true Arctiæ.
Natural Order—Lariæ. Larva, with sixteen feet, and furnished with various brushes, or fascicles of hair, on different parts of the body, but mostly on the anterior dorsal segments; it rolls itself in a ring when touched. ”