24 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



The stomodseum elongates and soon establishes communication 

 with the mid-gut, Figs. 44 and 48. It lies along the ventral side of the 

 embryo, just inside the test, and is joined, but is not inclosed, by the 

 layer of ectoderm, Fig. 49, std. The shell-gland becomes slightly 

 invaginated, Figs. 44 and 45, sg, but it very soon arches upward 

 preparatory to protruding laterally to form the mantle, Fig. 49. The 

 mid-gut becomes prolonged posteriorly, and the mass of cells from which 

 the cerebral ganglia are formed arch inward and form two pouches that 

 open at the surface. The pouches lie side by side, Fig. 46, and generally 

 open to the exterior together. Sometimes, however, they open between 

 different test-cells, Fig. 47, and seem to be only slightly connected with 

 each other. In other cases only one of the pouches opens to the exterior 

 and this opening may be small. In some cases there may be no opening. 

 The walls of the pouches are rather thick but each is composed of a 

 single layer of cells. 



The shell-gland folds out at the edges and most of the dorsal cells 

 assume the character of mantle cells. Only those near the edges remain 

 especially large and glandular. The formation of the mantle is accom- 

 panied by the formation of the pedal ganglia, Fig. 57, pg, as ectodermal 

 thickenings, and the formation of the otocysts, Fig. 56, as ectodermal 

 invaginations. The next stage shows the otocysts apparently completely 

 closed off, lying very close to the clefts that separate the mantle from 

 the body wall. 



The formation of the mantle and shell is accompanied by a slight 

 lateral compression and a corresponding dorso-ventral widening of the 

 embryo. The test-cells become very much thinner and the vacuoles 

 nearly or quite disappear, but the cilia remain as described and the 

 movements of swimming are not altered. Several new organs make 

 their appearance. 



Cleared whole mounts show some of the internal organs, but 

 throughout it is necessary to resort to reconstructions from the study of 

 serial sections to get the relationship of the different parts. 



The body of the embryo now lies between the valves of the shell and 

 these are inclosed in the test. The foot, Figs. 33 and 55, f, is formed 

 by a rapid increase of cells between the mid-gut and the stomodaeum. 

 The anterior adductor muscle, Figs. 33, 55 and 58, aa, is formed at a 

 corresponding time (about 85 hours) and consists of a few muscle fibres 

 which stretch from one shell-valve to the other. It is left, by the 

 growth of the embryo, separated from the rest of the body, except at its 



