1902.] CONKLIX — EMBRYOLOGY OF A BRACHIOPOD. 57 



side with the ectoderm, while laterally and on its ventral side it is 

 bounded by mesoderm (Figs. 43-52, 58 and 61-63). 



8. Coslom. — For a long time after the division of the ccelom into 

 right and left cavities by the flattening of the embryo and the 

 closure of the blastopore these sacs communicate with each other 

 both anteriorly and posteriorly (Figs. 43-47, 48-52, 53-55). In 

 still later stages these communications are closed by the practical 

 elimination of the coelom in the cephalic and peduncular regions 

 through the proliferation of mesenchyme cells (Figs. 56-63). 

 The coelom sacs, which are at first of nearly the same size 

 both anteriorly and posteriorly (Fig. 25), become much con- 

 stricted in the peduncular region while they still remain 

 large in the head and mantle regions (Fig. 26). In the latter 

 region they then become lobulated, often showing a trefoil condi- 

 tion (Figs. 27, 28), and with the further development of the dorsal 

 and ventral mantle folds a lobe of the coelom is sent into each of 

 these folds (Figs. 29-33). ^^^ the same time the coelom in the 

 cephalic and peduncular regions grows smaller, while that in the 

 mantle grows larger. Finally almost the entire coelom is con- 

 tained in the mantle, the portion in the head and peduncle being 

 very small (Figs. 2>^ and 56-63). The posterior limits of the 

 peduncular coelom is marked by a narrowing of the peduncle, 

 which probably represents the fourth ** segment" of Shipley. The 

 coelom however is never segmented though it may be constricted in 

 certain places. The constrictions shown in Figs. 27-31 are quite 

 constant in position and are connected with the extension of the 

 coelom into the mantle lobes, but they never coincide in position 

 with the superficial constrictions of the body (mantle furrows). In 

 a few abnormal larvae of the stage shown in Fig. 25 I have found 

 each coelom sac partially divided by mesenchyme cells into three 

 cavities. That these divisions, however, have no real importance 

 is shown by the fact that their number differs in different larvae and 

 is sometimes different on opposite sides of the same larva. 



In the early larval stages the enteron is in close contact with the 

 ectoderm on the dorsal side, while a collection of mesoderm cells 

 on the ventral side of the enteron separates the two coelom 

 sacs and may be considered the rudiment of a ventral mesentery 

 (Figs. 49, 50). 



In later stages the coelom is almost entirely obliterated, except 

 in the mantle, and consequently the enteron is surrounded by 



