214 



LAMELLIBRANCHIA TA . 



LAMELLIBRANCHIATA. 



The larvae of Laraellibranchiata have in a general way the same 

 characters as those of Gasteropods and Pteropods. A trochosphere 

 stage with a velum but without a shell is succeeded by a veliger 

 stage with a still more developed velum, a dorsal shell, and a ventral 

 foot. 



The segmentation is unequal, and in a general way like that of 

 Gasteropoda, but the specially characteristic Gasteropodan type with 

 four large yolk spheres is only known to occur in Pisidium, and a type 

 of segmentation similar to that of Anodon (p. 82) appears to be the 

 most frequent. 



There is an epibolic or embolic gastrula, but the further history 

 of the formation of the germinal layers has been worked out so 

 imperfectly, and for so few types, that it is not possible to make gene- 

 ral statements about it. What is known on this head is mentioned 

 in connection with the description of the development of special 

 types. 



The blastopore in some cases closes at the point where the anus 

 (Pisidium), and probably in other cases where the mouth, is eventually 

 formed. In Anodon it is stated to close at a point corresponding 

 neither with the mouth nor the anus, but on the dorsal surface ! 



The embryo assumes a somewhat oval form, and in the free 

 marine forms there appears very early in front of the mouth a well de- 

 veloped velum. This is formed according to Love'n from two papillse, 

 and takes the form of a circular ridge armed with long cilia. In the 



FIG. 117. THREE STAGES IN THE DEVELOPMENT OF CAEDIUM. (After Loven.) 

 hy. hypoblast ; b. foot ; m. mouth ; an. anus ; V. velum ; cm. anterior adductor 

 muscle. 



centre of the velar area there is usually present a single long 

 flagellum (fig. 117 B and C). The velum never becomes bilobed. 



In the later stages, after the development of the shell, the velum 

 becomes highly retractile and can be nearly completely withdrawn 

 within the mantle by special muscles. It forms the chief organ of 

 locomotion of the free larva. 



In some fresh- water forms, which have no free larval existence, the 

 velum is very much reduced (Anodon, Unio, Cyclas) or even aborted 

 (Pisidium). In these forms as well as in Teredo and probably other 



