MOLLUSCA 547 



the mantle, and under such conditions as occur in the formation of 

 pearls this general epithelium is capable of secreting any of the three 

 shell layers. 



In the Mollusca the development of the trochosphere takes place in 

 a fashion identical with that described for the annelid. In the diagram 

 given here for Patella, we see the completion of gastrulation and the 

 appearance of the ciliated rings of the trochosphere (Fig. 374 A); also 

 the single large cell which gives rise to the mesoderm. Then in 

 Fig. 374 B we see the early veliger with an internal organization 

 similar to the annelid, with apical organ, larval nephridia and proto- 

 troch. The figure shows, however, organs which are not present in 

 the annelid. On the dorsal side between the prototroch and the anus 

 the larval ectodermal epithelium forms the rudiment of the mantle 

 and even at this early age secretes the first shell. On the ventral side, 

 there is a prominence which is the foot (formed by the union of two 

 rudiments). The single mesoderm cell gives rise first of all to two 

 regular mesoderm bands ; and by the development of a cavity in each 

 of these, right and left coelomic sacs are formed ; then instead of seg- 

 menting as in the annelid, these largely break up into single cells, 

 some elongating and becoming muscle cells (Fig. 374 C). It is because 

 there is never any commencement of segmentation in the embryonic 

 mesoderm in molluscs that we have the strongest grounds for be- 

 lieving that molluscs never had segmented ancestors. The trocho- 

 sphere is followed by a second free-swimming stage, the veliger (Fig. 

 375), in which the prototroch develops into an organ, the velum, of 

 increased importance, which serves not only for locomotion but also 

 for feeding, the cilia creating a current which brings particles into 

 the mouth. In the veliger stage the foot increases in size and the 

 shell often becomes coiled in the Gasteropoda. 



Class AMPHINEURA 



Mollusca with an elongated, bilaterally symmetrical body, the mouth 

 and anus at opposite ends; with a head, without tentacles or eyes, 

 tucked under the mantle, which occupies the whole of the dorsal 

 surface, and contains various kinds of calcareous spicules imbedded 

 in cuticle, sometimes united to form continuous shells; a flattened 

 foot sometimes reduced ; a nervous system (Fig. 398 A) without definite 

 ganglia, the ganglion cells being evenly distributed along the length 

 of the nerve cords, and composed of a circumoesophageal commissure 

 and two pairs of longitudinal cords [pedal and palliovisceral), each 

 pair united by a posterior commissure dorsal to the rectum ; a radula; 

 usually a trochosphere larva. 



PoLYPLACOPHORA. Shore-living amphineura with flat foot which 



