58 LILLIE. [Vol. X. 



The actual function of the myocytes is the same as in other 

 animals : i.e., to carry out temporary movements of parts. 

 Some of them may be compared, though not perhaps homolo- 

 gized, with the retractors of the velum in Teredo for instance. 

 The pair shown in Fig. ^6 is doubtless that which functions 

 later on in the movement of the hooks of the shell. Before 

 the invagination of the larval mantle they all act as retractors 

 of the soft parts. 



3. Entodermal Organs. 

 We left the entoderm in the form of a small sac communi- 

 cating by a comparatively wide mouth, the blastopore, with 

 exterior (PI. V, Figs. 66, 68). In the next stage the blastopore 

 is practically closed by the forward growth of the cells of the 

 ventral plate area, and the entoderm is now represented by a 

 small clump of cells lying in contact with the ectoderm. The 

 dorsal and ventral lips of the blastopore meet, but do not fuse, 

 so that it is possible to recognize the anterior end of the blas- 

 topore throughout. The entoderm generally assumes the form 

 of a sac in the stage of Fig. 73 {cf. Fig. ']']), but at other times 

 no lumen is discernible. This is but an example of the usual 

 variability of rudimentary structures. 



4. Summary. 

 In this section we have followed the development of the 

 gastrula into the young larva. Before passing on to its trans- 

 formation into the glochidium I shall briefly describe the young 

 larva, comparing the names which I have used for the various 

 parts with those already in use, to serve as a point of departure 

 for the next section. In side view the young larva is roughly 

 quadrangular ; in transverse section, or end view, triangular 

 (Figs. 79, 80). The straight hinge-line extends along the 

 whole of the dorsal surface. Each valve of the shell covers 

 about two-thirds of its side ; the anterior, posterior, and ventral 

 edges of the shell are curved (Fig. 79), The cells lining the 

 shell are extremely flat ; so much so, that the nuclei produce 

 swellings. The ventral surface from the margin of the shell 

 is formed of large columnar cells. About the centre of the 



