410 INVERTEBEATA chap. 



shape from a large round opening to a narrow slit-like groove. Then 

 the edges of this groove approach one another and finally cohere in 

 the posterior part of the embryo. In the anterior portion of the groove 

 a pore opening into the archenteron persists for some time. Finally 

 this is closed, but a shallow pit is left, and in this same spot, at a later 

 date, the invagination to form the stomodaeum arises. In this way 

 a valuable landmark is created for the correlation of the region of 

 the larva with reference to the adult organs. 



On the opposite side of the larva from that on which the stomo- 

 daeum is situated — a side which we must regard as the dorsal side — 

 a depression appears in the form of a transverse groove which is the 

 beginning of the mantle-groove, and the embryo now takes on the 



Fig. 324. — Dorsal and ventral views of a larva of Terebratulina septerdrionalis. 

 (After Couklln.) 



A, dorsal view. B, ventral view, af, apical plate ; coe^, coe2, coe3, the three divisions of the 

 coelora on the left side ; d,in.f^ dorsal mantle-fold ; d.m.g, dorsal posterior mantle-groove ; ent, gut ; 

 stom^ rudiment of the stomodaeum ; v.m.f, ventral mantle-fold ; v.lii.g, ventral anterior mantle-groove. 



form of a top with a broad anterior and a narrow posterior region. 

 At the same time the communication between the gut and the 

 coelom becomes completely closed. 



In the dorsal region the mantle-fold appears as a fold or lip 

 overhanging the mantle-groove in front. This fold has a crescentic 

 form, being situated farthest forward in the middle portion, and 

 inclining downwards and backwards at the two sides. It soon extends 

 on to the ventral surface in the form of two backwardly directed 

 folds which meet in the mid-ventral line at an acute angle. The 

 point where they meet is situated just behind the spot where the last 

 trace of the blastopore was seen, and where the pit is found which 

 marks the site of the future stomodaeum (Fig. 324). 



When the mantle-fold has been completed, it will be observed 

 that there is a mantle-groove not only behind it but also in front of it. 

 The anterior mantle-groove marks off a " head segment," whilst 



