. Text-book of embryology. Embryology. 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 macromere, which buds off the micromeres which give rise to the ectoderm; instead of there being, as in all other MoUusca, four macromeres. 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- senda, one is much larger tha
. Text-book of embryology. Embryology. 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 macromere, which buds off the micromeres which give rise to the ectoderm; instead of there being, as in all other MoUusca, four macromeres. 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- senda, one is much larger than the rest. To the statement that the development of Pelecypoda, up to the vehger 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, ^^ l^p 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 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 (Kg. 280). A sagittal section through this extraordinary structure reveals inside it a saddle-shaped shell gland, a long narrow stomo- daeum leading up to a stomach, and a cerebral ganghon 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 ciha 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 Cyclas, Pisidium, etc., and the family of the Unionidae, where the early stages of development a
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