#spinecone

Although more highly organized than the Acephala, the snails are in some respects more primitive. The regions of the body—foot, visceral sac, head, and mantle—occur in all orders, although in each forms may occur in which one or another part is lost. As a rule, the foot is flattened ventrally to a creeping sole. In it may be distinguished anterior and posterior processes, the propodium and metapodium, a sharp lateral margin, the parapodium, and, above these, appendages or ridges, the epipodia. Inside the foot is usually a pedal gland.

The head bears the tentacles, a pair of muscular lobes or hollow retractile processes; a pair of primitive vesicular eyes, which usually lie at the basis of the tentacles, but may rise even to their tips. In many snails the eyes are on special stalks which (stylommatophorous Pulmonata) form a second pair of tentacles. The protrusion of the hollow tentacles is caused by an inflow of blood, their retraction by muscles attached to the tip which draw them in like a finger of a glove.

The mantle extends from the back forward over the body to near the beginning of the head. It covers the spacious mantle cavity, which in the water-breathing Prosobranchiata, etc., contains the gills (ctenidia) and opens outward by a large aperture under the margin of the mantle. The edge of the mantle may be produced into a long groove-like ciliated siphon, conveying water to and from the branchial chamber, and is of importance in determining the shape of the shell. In the terrestrial snails, branchiate respiration is replaced by pulmonate (Pulmonata, Cyclastoma), which is retained in many forms (Basommatophora) which have returned to an aquatic life. In place of the mantle cavity there is a sac filled with air, with a rich network of blood-vessels in its dorsal wall, and with only a small opening, the spiracle, on the right side. This lung was formerly thought to be the mantle cavity in which the ctenidium had degenerated, but development shows it to be an evagination arising in the mantle groove.

The visceral sac, by the great development of the gonads and liver, becomes very large. Since growth downwards is prevented by the muscular foot, the organs press towards the back, carrying before them the dorsal wall at the origin of the mantle folds, the line of least resistance. Some organs, like hind gut, nephridia and heart, may be pressed into the roof of the mantle cavity. When the visceral sac, as often occurs, becomes enormous, it does not stand directly upwards, but coils, usually from left to right, in a spiral. The older the animal the more the spiral coils and the larger the last or body whorl.

From the foregoing the shape of the shell is easily understood. As a secretion of the mantle and the visceral sc it takes the form of the latter. With slight development of the visceral sac it forms a flattened cone, or is slightiy coiled at the apex, as in the abalone. When the visceral sac is greatly elongate the shell is correspondingly an elongate cone. It is rarely irregularly coiled. It is usually coiled like a watch spring in one plane, or like a spiral staircase; in the latter case the shell is more or less conical and one can speak of its apex and base. In the middle of the base is usually a depression, the umbilicus. Sometimes the coils do not touch in the axis connecting umbilicus and apes, but usually they fuse into a calcareous pillar, the columella, around which the whorls pass.

The shell increases by additions from the mantle edge; and since this determines the aperture, the shell is marked with parallel lines of growth. The pigment is produced on the edge of the mantle, and passes into the shell as formed, causing its color pattern. When the siphon is present the shell shows a corresponding process. Thus are distinguished holostome shells with entire mouths and siphonostome shells, in which the anterior margin is drawn out in a groove.

A simple conical shell without further evidence is not proof of primitive structure. It may arise from the spiral form by degeneration, if the visceral sac be reduced. Thus the shells of Fissurella and Patella are to be explained, for the viscera here show the results of an earlier spiral twist.

In most places the union between shell and soft parts is not very firm, but the connection at the aperture is more intimate, while a muscle is attached to the columella at about its middle, the other end being inserted in the foot. This retracts the animal within the shell, the head and anterior part of the foot going first and then the metapodium, with its dorsal surface towards the aperture. Hence in many species this surlace secretes a door, or operculum, which closes the aperture when the body retracts. Since the aperture increases in size with growth, the operculum enlarges in a spiral manner, sometimes forming a spiral line on the outside, or on both surfaces. Land snails are usually without opercula, but in hibernation they can close the shell by a calcareous plate, the epiphragm. In the spring this separates from the shell and is lost.

In most gastropods the shell is coiled to the right, but in some species the whorls are constantly turned to the left, while reversed specimens occasionally occur in many species which are normally dextral.

There are usually two layers in the shell, an inner lamellar layer (not always present), which sometimes is highly iridescent, and an outer opaque, porcellanous layer, contains the pigment and may be covered by a horny cuticle. In rare cases mantle and shell are lacking, or the mantle is present but the shell is rudimentary and not visible externally because the mantle folds have grown over it. In these cases the visceral sac is not prominent. Since the shell-less forms possess a mantle and shell in the young, the adult conditions are explained by degeneration.

Only a few gastropods are bilaterally symmetrical. Usually the spiral twist of the visceral sac has resulted in a torsion of other parts from left to right, in which alimentary tract, nephridia, gills, heart, and nervous system take part. The intestine is bent in this way, the anus opening into the mantle chamber on the right side or the twisting may be continued so far as to double the intestine on itself, the anus being in the middle line in front, near the head (C). Nephridia, gills (with them the osphradia) and heart wander in company, so that the organs primitively belonging on the left side may be transferred to the right and vice-versa. With this there is a tendency to asymmetry and the loss of the organs (usually of the primitively left side). When the nervous system takes part in the twisting a crossing of the cerebrovisceral commissures takes place, known as streptoneury or chiastoneury.

The alimentary canal begins with a muscular region which in some groups is developed into a large protrusible proboscis. The pharynx, which follows, contains the tongue, a ventral ridge supported by one or more cartilages and corered by the lingual ribbon or radula (odendophore). The upper surface of the radula is armed with sharp, backwardly directed teeth arranged in transverse and longitudinal rows: these vary so in number, form, size, and arrangement that they are of value in classification. The radula is formed in the radula sac, which lies behind the tongue. From this it grows forward like a nail over its bed as fast as it is worn out in front. It is opposed in eating by a single median or a pair of lateral jaws (lacking in carnivorous forms).

The rest of the alimentary canal is convoluted, the anus being usually on the right side in front, in or beside the mantle chamber. Rarely it empties in the middle line behind. Esophagus, stomach, and intestine are slightly marked off from each other. The convolutions of the intestine are enveloped by the liver, which forms the chief part of the visceral sac. A pair of salivary glands empty into the pharynx, these in the Doliidae secreting a saliva containing 5 per cent. of free sulphuric acid.

The nervous system usually differs from that of other molluscs in that the pleural and parietal ganglia are separate. If the commissures be short, the ganglia are collected near the pharynx and, thus freed from the body torsion, are symmetrical. If the cerebrovisceral commissures be longer, the result is almost always streptoneury. Pleural and visceral ganglia hold their place, but the right parietal ganglion crosses above the intestine to the left side (hence called supraintestinal), while the left passes under the intestine to the right side (subintestinal), the cerebrovisceral commissures being twisted like the figure 8. The strong development of the pharynx is accompanied by buccal ganglia.

Gills, heart, and nephridia are best treated together. Certain genera (Haliotis, Fissurella, etc.) recall the Acephala in having the heart traversed by the intestine, the paired ctenidia, nephridia and nephridial ducts, and two auricles to the heart. As a rule the asymmetry induced by the torsion of the body has resulted in the loss of the ctenidium, osphradium, and nephridium of the primitively left side, and with the loss of one gill there is usually a loss of the corresponding auricle. Prosobranchs and Opisthobranchs are recognized accordingly as the gills are on the anterior or posterior part of the body. In the Opisthobranchs the ctenidia have been lost and are replaced by secondary gills on the back. Here the heart is in front of the gills; it receives blood from behind and forces it forward to the head by an aorta. In the Prosobranchs the heart has been twisted so that the auricle and the ctenidium are in front, while the aorta leads backwards. The nephridium, which communicates with the pericardium by a nephrostome, is usually saccular, its duct empties beside the anus.

The always unpaired sexual organs in some forms (Cyclobranchs and many Zygobranchs) empty into the nephridia and possibly the others utilize the rudiments of the right kidney. The sexual opening is almost always on the right side, beside the anus or in front of it on the head. Its position may be recognized in males and in hermaphroditic species by the grooved dermal fold, the penis. Occasionally this is separated from the genital pore, but is connected with it by a ciliated groove. The sexual organs are very variable in structure.

They show two extremes. On the one hand are completely dioecious species, on the other there may be complete hermaphroditism (many Tectibranchs, Pteropoda), in which the male and female organs are united throughout their extent. Intermediate stages occur; that of the pulmonates is shown in fig. 339.

The terrestrial snails lay their large tough-shelled eggs in damp earth; in the aquatic forms the eggs are laid in masses, usually gelatinous, each egg with a layer of albumen and a firm shell. Occasionally there is a kind of nest, as is the case with Ianthina which carries the mass of eggs, attached to the foot, about with it. A few gastropods are viviparous.

In the development the great constancy with which the veliger stage appears is noticeable. Most marine larvae swim at the surface by their velum before creeping at the bottom. But in those cases where the snail leaves the egg in the adult form the velum is usually developed in embryonic life, sometimes so strongly that the embryo rotates in the surrounding fluid.

Now, what were we talking about?

...Oh! Right! This thing. Spinecone.

Spinecones are, um, conical-shelled snails. Which are spiny. That’s good enough for me! It’s a simple but effective design. They also have 👀 emoji eyes, and I think that’s great.