130 



GASTROPODA. 



vr... 



It is only in comparatively few Gastropods that the Trochophore is 

 developed in such a pronounced manner as in Patella, this being no 

 doubt due to the fact that, in most Gastropods, a great part of the 

 development of the embryo takes place under the protection of the 

 eo-cshell or in the egg-capsule. The Trochophore stage is neverthe- 

 less to be found in all Gastropods, although it is more distinct in some 

 than in others. In these latter, as a rule, the larva attains free life 

 at a stage in which its shape has already undergone several modifica- 

 tions The later ontogenetic stages of Patella are not known, but 

 the larva, at the stage in which it is provided with a foot and a fairly 

 developed shell, still resembles the Trochophore, so that we may 

 assume that it does not undergo any further changes except those 



which are determined 

 by its transformation 

 into the adult. This 

 also seems to be the 

 case in Fissurella as 

 far as its develop- 

 ment is known 

 (Boutan, No. 12). 

 In this Gastropod, 

 the velum broadens 

 somewhat and as- 

 sumes a bilateral 

 form. This was also 

 already described in 

 the Lamellibranch 

 larva, and Fissurella 

 does actually show a 

 certain resemblance to the later stages of these forms (Fig. 17, p. 36), 

 if we leave out of consideration the shell which in the one case is 

 single and in the other bivalve. 



In the two last-named primitive Gastropods, the velum does not 



differ essentially from the Lamellibranch velum, but in most other 



forms its shape becomes modified in a manner specially characteristic 



of the Gastropoda. The bilateral development of the velum which 



is already indicated in Fissurella is, in those Prosobranchs the eggs 



of which are rich in yolk (Neritina, Vermetvs, Fulgar), evident when 



the velum first appears as a rudiment, This organ appears in the 



embryo at first in the form of two specially marked rows of cells 



(Neritrruc) or two curved ridges which unite only later to form the 



v. 



Fig 5i-Velige?- larva with four-lobed velum (after 

 McMukrich). f foot ; m, oral aperture ; f-PjJ^gj 



p , post-oral ciliated ring ; s, shell ; t, tentacle wren 

 eye at its base ; v, velum. 



