Gastropoda

“ Euthyneurous Gastropoda, probably derived from ancestral forms similar to the Tectibranchiate Opisthobranchia by adaptation to a terrestrial life. The ctenidium is atrophied, and the edge of the mantle-skirt is fused to the dorsal integument by concrescence, except at one point which forms the aperture of the mantle-chamber, thus converted into a nearly closed sac. Air is admitted to this sac for respiratory and hydrostatic purposes, and it thus becomes a lung. An operculum is present only in Amphibola; a contrast being thus afforded with the operculate pulmonate Streptoneura (Cyclostoma, &c.) ”

“ Very usually, but not universally, the metapodium carries an operculum. The division of the foot into lobes is a simple case of that much greater elaboration or breaking up into processes and regions which it undergoes in the class Cephalopoda. Even among some Gastropoda (viz. the Opisthobranchia) we find the lobation of the foot still further carried out by the development of lateral lobes, the parapodia, whilst there are many Pectinibranchia, on the other hand, in which the foot has a simple oblong form without any trace of lobes. ”

“ One of the most intriguing patterns in mammalian biogeography is the “island rule”, which states that colonising species have a tendency to converge in body size, with larger species evolving decreased sizes and smaller species increased sizes. It has recently been suggested that an analogous pattern holds for the colonisation of the deep-sea benthos by marine Gastropoda. In particular, a pioneering study showed that gastropods from the Western Atlantic showed the same graded trend from dwarfism to gigantism that is evident in island endemic mammals. ”

“ The chief types of Mollusca were already differentiated at the beginning of the geological record, and the metamerism which occurs in the Cephalopoda has been evolved within the limits of that class.
External Characters.—The characteristic organs of Mollusca are the mantle and shell, the foot, the ctenidia and the radula, of which all but the last are external. The original form was bilaterally symmetrical, and this symmetry is retained in all the classes except the Gastropoda. At the anterior end the head is differentiated ”

“ In the majority of cases, however, in which there is a small amount of food yolk and no special arrangements for parental care, a larva is formed. No better group than the Mollusca can be taken to illustrate this point, for in them we find every kind of development from the completely embryonic development of the Cephalopoda, with their large heavily-yolked eggs, to the development of most marine Lamellibranchiata and many Gastropoda, in which the embryonic period is short and there is a long larval development. ”

“ The structure of the Mollusca in the greater number of cases agrees with the hypothesis that the primitive form was unsegmented, and therefore had but one pair of coelomic ducts and one pair of nephridia. In existing forms the latter disappear in the adult. In the most primitive forms of several classes there are no distinct genital ducts, the gonads when mature discharging into or through the kidneys. Among the Gastropoda, in the Aspidobranchia, there is no genital duct, and the gonad opens into the right kidney ”

“ No opening into the body-cavity has been made; the organs which lie in the coiled visceral hump show through its transparent walls. The head is seen in front resting on the foot and carrying a median non-retractile snout or rostrum, and a pair of cephalic tentacles at the base of each of which is an eye. In many Gastropoda the eyes are not thus sessile but raised upon special eye-tentacles (figs. 25, 56) . To the right of the head is seen the muscular penis p, close to the termination of the vas deferens (spermatic duct) vd. The testis t occupies a median position in the coiled visceral mass. ”

“ Devonian to Recent.
3. Gastropoda—the snails and slugs; slugs are without shells, snails have a single-valved shell which is typically coiled; common fossils in Paleozoic, Mesozoic, and Cenozoic rocks. Cambrian to Recent.
4. Pelecypoda—clams, mussels, oysters, scallops; shells composed of two valves, usually, but not always, of equal size; common fossils, especially in Mesozoic and Cenozoic rocks. Cambrian to Recent.
5. Cephalopoda—squids, octopuses, the pearly nautilus, and the ammonoids (extinct) ; shell of one valve, usually coiled and partitioned by septa ”

“ Discussion Monophyly of Neogastropoda
Recent molecular phylogenetic analyses (e.g. [20] ) are challenging our view of gastropod systematics and many traditional groups (e.g. Pulmonata) , widely accepted as monophyletic, are now seriously questioned [ [39] - [40] . As a result, a general perception is emerging that many morphological characters previously used to infer phylogenetic relationships among gastropod main lineages may be homoplasious [ [10] , [20] , and that gastropod phylogeny needs to be revisited. ”

“ I differ from the Gasteropoda (whether they be terrestrial or aquatic) in that I and my near relations are exclusively aquatic and of a sedentary life, which makes the protection of two encompassing shells necessary. These shells are secreted by my mantle lobes, and are united by a ligament which tends to make the valves ‘gape’ for water and food and by two contracting muscles which close them in danger. I have a degenerate brain and no eyes. My mouth has neither jaw nor teeth, but possesses nervous lips covered with cilia, the vibration of which carries food-laden water to my mouth. ”

“ In the class of Cephala, to which our snail belongs, there is the sub-class of Gasteropoda, or stomach-footed, because on the ventral side of the body a sole-like disc or foot exists, by the wave-like expansions and contractions of which the animal progresses.
In this sub-class there is a division according to their having or not having an operculum, or means of closing and protecting the orifice of the shell. Most gasteropods which live in water have this; most which live on land (only [Pg 25] two exceptions in British molluscs) have not. ”

“ Gastropods.
The shell of the Pulmonata, though always light and delicate, is in many cases a well-developed spiral “house” into which the creature can withdraw itself; and, although the foot possesses no operculum, yet in Helix the aperture of the shell is closed in the winter by a complete lid, the “hybernaculum” more or less calcareous in nature, which is secreted by the foot. In Clausilia a peculiar modification of this lid exists permanently in the adult, attached by an elastic stalk to the mouth of the shell, and known as the “clausilium.” ”

“ Some are herbivorous, and others carnivorous; many prey on living, and others on decaying animal and vegetable substances. [378] As « 420 » in a fossil state the shells alone remain to afford any clue as to the structure and economy of the originals, characters have been sought for, by which the fluviatile or marine nature, and the carnivorous or herbivorous habits of the living mollusca may be determined. As a general rule, it will be found, that the shells of terrestrial and fresh-water Gasteropoda have the aperture entire, as in the Garden Snail, and in the fossil shell, Lign. 133, fig. ”

“ The animal which has been seen to hatch from the egg of entoconcha is a young gasteropod, which, like the young of ordinary forms, has a spiral shell, a muscular locomotor foot, tentacles, eyes, hearing organs, and a nervous system like that of other gasteropods at the same stage of growth. The digestive tract is divided into regions, like those of an ordinary gasteropod, and the young entoconcha has no peculiarities to indicate that the structure and habits of the adult are to be in any way strange or unusual ”

“ Forms adapted to terrestrial life and to aerial respiration occur in various divisions of Gastropods, and do not constitute a single homogeneous group. Thus the Helicinidae, which are terrestrial, are now placed among the Aspidobranchia. In these there are neither branchia nor osphradium, and the pallial chamber which retains its large opening serves as a lung. Degeneration of the shell occurs in some members of the order. It is largely covered by the mantle in some Fissurellidae, is entirely internal in Pupilia and absent in Titiscaniidae. ”

“ This gizzard has not the same function in all cases, for whereas in some forms (e. g., in Bowerbankia) it is lined with horny projections and is a powerful crushing organ, in others (e. g., in Hislopia or Victorella) it acts as an antechamber in which food can be preserved without being crushed until it is required for digestion, or rough indigestible particles can be retained which would injure the delicate walls of the stomach.
Digestion takes place mainly in the stomach, the walls of which are of a glandular nature. ”

“ The animals do not glide like most other molluscs, but scramble along like the Strombs, the form of their foot being small, divided into a front, expanded, and a hind, tapering portion admirably adapted to the nature of the ground on which they live, which usually consists of broken and dead shells.
Cases 49–52. The “Wing-shells” (Strombidæ) are the largest of the Gastropods with a proboscis or non-retractile snout. They do not crawl like most other Gastropods, but progress by a sort of hopping movement. They act as scavengers, feeding on decomposing animal matter. ”

“ The Gasteropoda breathe both by gills and lungs; examples of the gill-breathing kind are seen in the Whelks (Buccinum) (Fig. 52) often picked up on the sea-shore while the garden-snail will represent the lung-breathing kind. The beautiful Carinaria, with its delicate propeller, is a highly specialized gill-breather. The Cephalopoda are the most highly organized of the Mollusca. The Cuttle-fishes (Fig. 54) , with their long arms, are familiar to all who have read the "Toilers of the Sea." ”

“ All Cephalopoda. Gasteropoda Amphineura (except Neomeniidae) . Gasteropoda Prosobranchiata (except Valvata and some species of Marsenia) . Scaphopoda. Many Pelecypoda.
[134]
2. The sexes are united in
Gasteropoda Opisthobranchiata. Gasteropoda Pulmonata. Certain Pelecypoda. [254]
In the dioecious Mollusca, sexual union is the rule, but is by no means universal. In some instances,—e.g. Vermetus, Magilus, Patella, Haliotis, Crepidula, Chiton, the Scaphopoda—the form and habits of the animal do not admit of it; in others (many Trochus) a male copulative organ is wanting. ”

“ In some Phyllopoda (Apus) egg-sacs are formed by modification of certain of the thoracic feet. The eggs are retained between the valves of the shell in some Phyllopoda and in the Cladocera and Ostracoda, and they lie in the mantle cavity in the Cirripedia. In the Copepoda they are agglutinated together into masses attached to the body of the female. Among the Malacostraca some Schizopoda, the Cumacea, Tanaidacea, Isopoda and Amphipoda (sometimes grouped all together as Peracarida) have a marsupium or brood-pouch formed by overlapping plates attached to the bases of some of the thoracic legs. ”

“ Some, like many Cephalopods and the Pteropods, are pelagic or free-swimming; others creep or lie on the sea bottom. Some are littoral, living between tide-marks; others are found at very various depths, up to 2800 fathoms. A few species have invaded the fresh waters, while the pulmonate and terrestrial Gastropods are distributed over the whole surface of the land in all latitudes and to a height of 15,000 ft. As a rule Molluscs are free and more or less active, but many Lamellibranchs are sedentary, and a few of these and of Gastropods are permanently fixed to their habitat. ”

“ The Gasteropoda are extremely numerous; most of them have univalve shells, but many of them have none, as the Garden Slug (Limax, fig. 35) . The Whelk (Buccinum undatum) , Perriwinkle (Littorina littoria) , Garden Snail (Helix aspersa, fig. 35) , and the Wentle-trap (Scalaria, fig. 36) , are the most familiar examples of this class. These Mollusca walk by means of the [Pg 258] mantle, which is muscular, and capable of alternate contraction and expansion; they breathe by means of lungs on their back beneath the shell, and to which there is an opening in their side. ”

“ In most Gasteropoda the liver consists of two distinct lobes, between which are embedded the stomach and part of the intestine. In many Nudibranchiata the liver becomes ‘diffused’ or broken up into a number of small diverticula or glands connecting with the stomach and intestine. The so-called cerata or dorsal lobes in the Aeolididae are in effect an external liver, the removal of which to the outside of the body gives the creature additional stomach-room. ”

“ The “introvert” in these Gastropods is not the pharynx as in the Chaetopod worms, but a prae-oral structure, its apical limit being formed by the true lips and jaws, whilst the apical limit of the Chaetopod’s introvert is formed by the jaws placed at the junction of pharynx and oesophagus, so that the Chaetopod’s introvert is part of the stomodaeum or fore-gut, whilst that of the Gastropod is external to the alimentary canal altogether, being in front of the mouth, not behind it, as is the Chaetopod’s. ”

“ I must pass by many groups and orders to more aberrant types, represented by the naked-gilled orders, Opisthobranchiata and Nudibranchiata. These gastropods constitute a large sub-order of extremely beautiful molluscs, remarkable in shape, and often brilliant in colour. The distinguishing character of these typical forms consists in the peculiar nature and situation of their breathing organs, which are exposed on the back of the animal or around the anterior part, and are not protected by the mantle. ”