DEVELOPMENT OF THE EARLY STAGE. 



51 



The shell forms in exactly the same way as in the marine Lamellibranchs 

 (Figs. 14, p. 28, and 15, p. 31) and in Cyclas (Fig. 19, p. 40), except that 

 the shell-gland is specially large and appears very early. This early develop- 

 ment of the shell, as suggested by GOETTE, is probably due to the great im- 

 portance of the shell to the larva in its free life which we shall discuss later, 

 and, in the same way, we may explain the degeneration of the intestine by 

 the parasitic life of the larva, in consequence of which the intestine is not 

 required to fulfil its ordinary functions until a late stage. 



In spite of the highly modified character of the larvae of the 

 Unionidae, we are able to make a comparison between their organs 

 and those of the typical Trochophore larva. Apart from the ento- 

 dermal and mesodermal parts which have already been mentioned 



FIG. 23. A-C, embryos of A nodonta piscinalis, median optical section (after GOETTE) ; 

 e, entoderm-rudiment (archenteron) ; m, mesoderm ; rk, polar bodies; s, shell; sd, 

 shell-gland ; sm, adductor muscle ; w, posterior ciliated area [ventral plate], 



(pp. 27 and 28), the most striking feature is the ciliated area (Figs. 

 22-24, w), which is evidently the last vestige of the ciliation of the 

 free-swimming larva. This ciliated area, termed the ventral plate, 

 does not, as was at first supposed, correspond to a remains of the 

 velum, but represents the whole ventral surface plus the posterior 

 end of the body, and is therefore rather to be compared with the 

 ventral ciliation or with the anal ciliated area of the Trochophore 



spheres resulting from the first two divisions, that is to say, neither the first 

 nor the second cleavage-plane divides the egg into an animal cell and a vege- 

 tative cell as stated by RABL. LILLIE further finds that the small archenteron 

 is formed before the invagination of the shell-gland, which latter, however, 

 soon eclipses the former. Of the two groups of mesoderm-cells represented 

 in Figs. 22 and 23 A, those above the shell-gland would correspond with 

 LILLIE'S larval mesenchyme, while those below this structure and behind the. 

 blastopore, e.g., the large cell in Fig. 23 A, represent the adult mesoderm 

 which forms teloblastically from the pair of mesoderm-cells. For more 

 detailed figures see LILLIE'S paper. ED.] 



