MOLLUSCA. 



259 



is mentioned in connection with the description of the develop- 

 ment 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-developed velum. This is formed according to Lov6n from 

 two papillae, and takes the form of a circular ridge armed with 

 long cilia. In the centre of the velar area there is usually 



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

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

 muscle. 



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 withdrawn within the mantle by special muscles.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 exist- 

 ence, the velum is very much reduced (Anodon, Unio, Cyclas) or 

 even aborted (Pisidium). In these forms as well as in Teredo 

 and probably other marine forms (e.g. Ostrea) the central flagel- 

 lum is absent. It has been suggested by Lovdn, though without 

 any direct evidence, that the labial tentacles of adult Lamelli- 

 branchiata are the remains of the velum. The velar area is in 

 any case the only representative of the head. In some marine 

 forms a general covering of cilia arises before the formation of 



17-2 



