26 



THE MOLLUSC A 



FIG. 1-J. 



Trochosphere of Patella, sagittal section, bl, blasto- 

 pore ; me, mesoderm ; sh.g, shell-gland ; the two dorsal, 

 ve, and the two ventral ciliated cells are the velar 

 cells. (After Patten.) 



An invagination of the ectoderm at the place where the 

 blastopore closed, or surrounding the blastopore if it remains open, 

 places the digestive cavity (enteron) of the gastrula in communica- 

 tion with the exterior. This 

 invagination constitutes the 

 stomodaeum, from which the 

 pharynx and oesophagus 

 with all their accessory ap- 

 paratus, the salivary glands, 

 radula, etc., are formed. 

 Thus the blastopore, if it 

 remains open, does not 

 become the mouth of the 

 adult. Paludina, however, 

 is an exception, in that 

 the whole of the blasto- 

 pore remains open and be- 

 comes the anus (Ray Lan- 

 kester), whilst the stomo- 

 daeum is formed at the 

 anterior end of the embryo 

 (Fig. 110, F, m). Later, in 

 Molluscs in general, the proctodaeum is formed as a very short anal 

 invagination, at the posterior end of the original blastoporic groove. 

 Its position is generally indicated by the presence of two prominent 

 ectodermic cells, and it perforates the posterior part of the archen- 

 teron, establishing a com- 

 munication between the in- 

 testine and the exterior. 



Ectodermic Organs. The 

 embryo, then, has an endo- 

 dermic digestive cavity and 

 a general ectodermic en- 

 velope from which the 

 oesophagus and the anal 

 invagination are derived. 

 But the ectoderm is prin- 

 cipally concerned in the 

 production of the permanent 

 tegumentary organs properly 

 so called, such as the foot, 

 the mantle, and the ctenidia, 

 and those organs which, al- 

 though deeply placed, originate from the surface, such as the nervous 

 system and the organs of sensation. The ectoderm further gives 

 rise to the embryonic locomotory organs which result from a special 



FIG. 13. 



Trochosphere of Dreissensia, median sagittal section. 

 bl, blastopore; fl, flagellum ; in, intestine; p.a.c, 

 post-anal cilia ; sh, shell ; vc, velum. (After Meiseii- 

 heimer.) 



