xn PHYLUM MOLLUSCA 735 



stage the shell (sh.) increases in size, loses its simple form, and 

 begins to develop a spiral. A cleft-like depression in the border 

 of the mantle on the right-hand side forms the rudiment of the 

 mantle-cavity in which, later, the gills are developed. The anus 

 when it first appears may be symmetrically placed, but later 

 becomes shifted to the right side and forwards as well as dorsally. 

 A pair of larval nephridia are developed, having a remarkable 

 resemblance to the larval nephridia of the trochophore of Poly- 

 chseta, but of mesodermal origin. Each consists of a longer or 

 shorter tube, usually intra-cellular, opening on the exterior at one 

 end, and at the other terminating in one or several solenocytes. 

 The foot (/.) may attain a considerable development during the 

 veliger stage, and on its posterior and dorsal part appears the 

 operculum. Two little processes on the velar area develop into 

 the tentacles (tent.), and the eyes (eye) appear at their bases. As 

 the foot and other organs advance in development the velum 

 decreases in size and gradually aborts, but in some cases a 

 portion of it persists as the subtentacular lobes or labial tentacles 

 in the neighbourhood of the mouth. 



In the Pulmonata the velum is not well developed, except in 

 Oncidium, though the trochophore stage is well marked. 



The young Gastropod is at first bilaterally symmetrical ; the 

 prevailing asymmetry is the result of unequal growth of the two 

 sides of the body. In the majority of cases it is the left side that 

 grows more actively than the right, a result of which is that the 

 posterior parts the anus and the region surrounding it are dis- 

 placed forwards towards the right, the space between the anus and 

 the mouth on that side undergoing little or no increase in length. 

 In the Opisthobranchia and the Pulmonata the anus with the 

 mantle-cavity and its contents become displaced forwards towards 

 the anterior end ; in most of the Streptoneura the anus, &c., in 

 their displacement forward pass beyond the middle line, one of the 

 most striking effects of which is the crossing of the pleuro- visceral 

 connectives already referred to. 



Ethology and Distribution. Only a few aberrant families of 

 Gastropoda are parasites. Most are aquatic, all the most primitive 

 forms being' inhabitants of the sea. Of the marine families the 

 majority move by creeping over the sea-bottom, some burrowing 

 in mud or sand, some in solid rock ; some are able to float in 

 a reversed position, adhering to frothy mucus secreted by the 

 glands of the foot ; certain exceptional forms such as Vermetus 

 are fixed in the adult condition by the substance of the shell. 

 A few families the Heteropoda and the Pteropoda are specially 

 modified for a pelagic mode of existence, and swim through the 

 water by flapping movements of the lobes of the foot, which act 

 as fins. Gastropods are found in the ocean at considerable 

 depths up to nearly 3,000 fathoms. Many forms, however, are 



3 A 2 



