In dextral Gastropods the only structure found on the topographically right side of the rectum is the genital duct.
In the most primitive existing Gastropods the gonad opens into the right kidney (Patellidae, Trochidae, Fissurellidae).
Triforis, Physa, Clausilia are examples of sinistral Gastropods, but reversal also occurs as an individual variation among forms normally dextral.
- These are the most primitive Gastropods, retaining to a great degree the original symmetry of the FIG.
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.
Supposing the tube to be completely introverted and to commence its eversion, we then find that eversion may take place, either by a forward movement of the side of the tube near its attached base, as in the proboscis of the Nemertine worms, the pharynx of Chaetopods and the eye-tentacle of Gastropods, or by a forward movement of the inverted apex of the tube, as in the proboscis of the Rhabdocoel Planarians, and in that of Gastropods here under consideration.
The introverted rostrum of the Pectinibranch Gastropods presents in contrast to these a limited range of FIG.
Spengel showed that the parabranchia of Gastropods is the typical olfactory organ or osphradium in a highly developed condition.
He fully recognized, however, the similarity of Pteropods to Gastropods in their general asymmetry and in the torsion of the visceral mass in Limacinidae.
It is now understood that they are Euthyneurous Gastropods adapted to natatory locomotion and pelagic life.
In most other Mollusca (Anisopleurous Gastropods, Pteropods and Conchifera) there is a want of such continuity; the primitive shell-sac contributes no factor to the permanent shell, or only a very minute FIG.
The external form of the embryo goes through the same changes as in other Gastropods, and is not, as was held previously to Lankester's observations, exceptional.
The loss of the power to coil, observed in the terminals of many declining series of gastropods from the Cambrian to the present time, and the similar loss of power among Natiloidea and Ammonoidea of many genetic series, as well as the ostraean form assumed by various declining series of pelecypods and by some brachiopods, may be cited as examples.
This is not due to environmental conditions solely, because senescent branches of normal progressive groups are found in all geologic horizons, beginning, for gastropods, in the Lower Cambrian.
Whereas among many ammonites and gastropods smooth ness of the shell, following upon an ornamental youthful Condition, is generally a symptom of decline, among many other invertebrates and vertebrates, as C. E.
Most of the fossils of the bess are shells of terrestrial gastropods, but bones of land mammals are also found in not a few places.
As the axis of the ctenidium lies by the side of the body, and is very frequently connate with the body, as so often happens in Gastropods also, we find it convenient to speak of the two plate-like structures formed on each ctenidial axis as the outer and the inner gill-plate; each of these is composed of two lamellae, an outer (the reflected) and an adaxial in the case of the outer gill plate, and an adaxial and an inner (the reflected) in the case of the inner gill-plate.
Fr, g, It does not contain blood or communicate directly with the bloodsystem; this isolation of the pericardium we have noted already in Gastropods and Cephalopods.
In them the foot has a flat ventral surface used for creeping, as in Gastropods, the byssus gland is but slightly developed, the pleural ganglia are distinct, there is a relic of the pharyngeal cavity, in some forms with a pair of glandular sacs, the gonads retain their primitive connexion with the renal cavities, and the otocysts are open.
The foot is a muscular mass without cuticle or skeleton, excepting certain cuticular structures such as the byssus of Lamellibranchs and the operculum of Gastropods, which do not aid in locomotion.
In more specialized forms a posterior aorta passes backwards from the ventricle, as in Gastropods and the majority of Lamellibranchs.
In Cephalopods and the majority of Gastropods copulation occurs.
As a rule no parental care is exhibited, but incubation of the developing ova within some part of the parental body, or receptacles attached to the parent, occurs in some Lamellibranchs, some Gastropods, and in Argonauta among the Cephalopods.
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 latter is confined to certain Gastropods which live in Echinoderms and are extremely degenerate in structure.
Protective resemblance is exhibited by some Nudibranch Gastropods which have assumed the colour and appearance of their habitat.
Amongst the orbweavers of the family Argyopidae there are species belonging to the genera Cyclosa and Cyrtophora which closely resemble small snail-like gastropods as they cling to the underside of leaves with their legs drawn up. Other members of the same family - like Araneus coccinella, and Paraplectana thorntoni- imitate beetles of the family Coccinellidae which are known to be distasteful; and certain genera of the family Salticidae (Homalattus and Rhanis) closely resemble small hard-shelled beetles.
Their bilateral symmetry is obviously to be regarded as primitive, and the nervous system shows an original condition from which that of the asymmetrical twisted Gastropods can be derived.
On this view then the Aplacophora are more primitive than the Polyplacophora in the relations of coelom, gonad and coelomoducts; and the genital ducts of the Chitons have arisen either by metameric repetition within the group, or by the gradual loss of an original connexion between the generative sac and the renal tube, as in Lamellibranchs and Gastropods, the generative sac acquiring a separate duct and opening to the exterior on each side.
In this way the ocellus becomes a true eye, very similar in plan to the eyes of Gastropods and other molluscs.
The gonad, whose position has already been mentioned, is divided into transverse lobes; its duct is anterior and single, and diverges to the right to open into the right kidney as in primitive Gastropods and Lamellibranchs.