IX 



MOLLUSCA 



349 



The only point which requires some comment is the description 

 given by the earlier workers of the segmentation stages. Thus 

 Horst (1882) and Hatschek (1883) both describe the endoderm as 

 represented by one huge ruacromere, which buds off the micromeres 

 which give rise to the ectoderm ; instead of there being, as in all 

 other Mollusca, four rnacromeres. There is strong ground for 

 believing that this is a misinterpretation, and that in all cases four 

 macromeres are really formed, but that, as is the case with Dreis- 

 sensia, one is much larger than the rest. 



To the statement that the development of Pelecypoda, up to the 

 veliger stage, pursues a uniform course in all genera, two marked 

 exceptions must be made. The first of these concerns the group of 

 the Protobranchiata, including 

 the genera Nucula, Leda, 

 Yoldia, etc., whose develop- 

 ment has been studied by 

 Drew (1899, 1901). 



In this group the velum 

 acquires enormous dimensions, 

 and consists of circles of large 

 vacuolated cells placed one 

 above the other, forming a 

 barrel-shaped structure. The a'dd.c 

 first and last circles bear 

 numerous small cilia all over 

 their surface, and the central 

 three circles have each a narrow 

 band of long cilia (Fig. 280). 



A sagittal section through this 



FIG. 279. The late Veliger larva of Ostrea 

 rin/hiiaiia, viewed from the side. (After 

 Stafford.) 



iidd.a, anterior adductor muscle ; mkl.p, posterior 

 adductor muscle; In; rudiments of gills; /, Coot ; 7/c/i, 

 lobes of liver ; ot, otocyst ; V, retracted velum. 



extraordinary structure reveals 

 inside it a saddle-shaped shell 

 gland, a long narrow storno- 

 daeum leading up to a stomach, 



and a cerebral ganglion arising in Yoldia as a pit in front of the 

 apical plate (Fig. 281). The foot appears later, and when the meta- 

 morphosis occurs and the velar cells are cast away, the cilia covering 

 the foot are sufficiently powerful to enable the animal to glide over 

 the mud in which it lives before any burrowing movements are 

 carried out. The general plan of the development is therefore the 

 same as in Dreissensia. 



At the other pole of variation are freshwater forms like Cydas, 

 Pisidium, etc., and the family of the Unionidae, where the early 

 stages of development are passed between the lamellae of the gill of 

 the mother, and where is therefore no free-swimming stage and 

 neither prototroch nor velum is developed. 



In Ziegler's account of the development of Cydas (1885) it is 

 stated that there is only one large macromere from which all the 

 micromeres are budded off. As we pointed out above, this is probably 



