How to use Gastropoda in a sentence
The peculiarities of the Gastropoda are all due to the torsion of the shell and body.
For a discussion of the relationship of the Gastropoda to the remaining classes of the phylum, see MoLLUSCA.
The Gastropoda are mainly characterized by a loss of symmetry, produced by torsion of the visceral sac. This torsion may be resolved into two successive movements.
The essential feature of the asymmetry of Gastropoda is the atrophy or disappearance of the primitively left half of the circumanal complex (the right half in sinistral forms), including the gill, the auricle, the osphradium, the hypobranchial gland and the kidney.
This conclusion has shown that the Euthyneura do not represent an archaic form of Gastropoda, but are themselves derived from streptoneurous forms. The difference between the two sub-classes has been shown to be slight; certain of the more archaic Tectibranchia (Actaeon) and Pulmonata (Chilina) still have the visceral commissure long and not untwisted.Advertisement
This condition, probably an archaic one, distinguishes the Aspidobranchia from other Gastropoda.
Many Gastropoda deposit their eggs, after fertilization, enclosed in capsules; others, as Paludina, are viviparous; others, again, as the Zygobranchia, agree with the Lamellibranch Conchifera (the bivalves) in having simple exits for the ova without glandular walls, and therefore discharge their eggs unenclosed in capsules freely into the sea-water; such unencapsuled eggs are merely enclosed each in its own delicate chorion.
Euthyneurous Gastropoda, probably derived from ancestral forms similar to the Tectibranchiate Opisthobranchia by adaptation to a terrestrial life.
Here we need only further draw attention to the osphradium, discovered by Lacaze-Duthiers, and shown by Spengel to agree in its innervation with that organ in all other Gastropoda.
As in other Gastropoda Anisopleura, this shell-sac may abnormally develop a plug of chitinous matter, but normally it flattens out and disappears, whilst the cap-like rudiment of the permanent shell is shed out from the dome-like surface of the visceral hump, in the centre of which the shell-sac existed for a brief period.Advertisement
Similar larval nephridia occur in other Gastropoda.
For the morphology and classification of snails, see Gastropoda.
The foot is commonly a simple cylindrical or ploughshare-shaped organ, used for boring in sand and mud, and more rarely presents a crawling disk similar to that of Gastropoda; in some forms it is aborted.
This is not an unusual arrangement in Lamellibranchs, and a similar disposition occurs in some Gastropoda (Haliotis).
As thus purified by successive advances of embryological research, the Mollusca were reduced to the Cuvierian classes of Cephalopoda, Pteropoda, Gastropoda and Acephala.Advertisement
The Chitons and their allies were placed under the Gastropoda, as a distinct branch called Isopleura, and for the Acephala de Blainville's name Lamellibranchia was substituted.
The Chitonidae and the Aplacophora are now separated from the Gastropoda and raised to the rank of a distinct class, under the name of Amphineura.
On the other hand, Boas and Pelseneer have shown that the Pteropoda have nothing to do with the Cephalopoda, but are Gastropoda modified for a pelagic life; they are therefore now united with the Gastropoda.
The Lamellibranchia are no longer regarded as a distinct branch in contrast to the remaining Mollusca; according to Pelseneer they are allied to the Gastropoda and Scaphopoda, all three classes being derived from a common hypothetical ancestor, called Prorhipidoglossum.
Among the Gastropoda, in the Aspidobranchia, there is no genital duct, and the gonad opens into the right kidney; in the more modified forms the left kidney alone is functional, the right has been converted into the genital duct.Advertisement
The latter are represented in the Upper Cambrian formations, together with Lamellibranchia and Gastropoda, and there are no earlier Molluscan fossils than these.
On the other hand, there is no evidence that the forms which show no metamerism, such as the Gastropoda, are descended from metameric ancestors.
The original form was bilaterally symmetrical, and this symmetry is retained in all the classes except the Gastropoda.
It was present in the ancestral mollusc, occurs in nearly all archaic types, and is only absent in the most specialized forms, in which it has evidently been lost; these forms are certain Neomeniomorpha, all the Lamellibranchia, various degenerate Gastropoda, and the Cirrhoteuthidae among Cephalopods.
In the Gastropoda the muscular tissue of the buccal mass is coloured red by haemoglobin.Advertisement
Hermaphroditism is secondary, and occurs in one sub-class of Gastropoda, in some Lamellibranchs, and in one sub-order of Amphineura.
True viviparity, that is the development of the ova within the oviduct, is very rare, occurring only in one case among the Amphineura and in some aquatic and pulmonate Gastropoda.
Originally the Chitons were placed with the limpets, Patella, in Cuvier's Cyclobranchia, an order of the Gastropoda.
At first supposed to be tubicolous Annelids, Dentalium and its allies were afterwards placed among the Gastropoda, to which recent authorities consider them to be closely related.
The nervous system resembles that of Gastropoda and Lamellibranchia.Advertisement
In Polyplacophora there are eight on each side (8.I.8); in Scaphopoda two on each side (2.I.2); in almost all Cephalopoda three on each side (3.I.3); in Gastropoda the number varies very much in different subdivisions.
In the majority of Gastropoda one gill and one auricle are lost.
In the larvae of several Gastropoda and Lamellibranchia occur excretory organs which have the characters of true nephridia.