SIPUNCULIDS 361 



finally fuse in the lateral lines ; furthermore, a complete 

 fusion takes place with the head-plate (Fig. 158^ and F). 

 An ectodermal invagination at the posterior dorsal part of 

 the larva produces the hind-gut, and fuses with the entoderm. 

 The deep and voluminous fundament of the fore-gut now 

 does the same. Two invaginations make their appearance 

 on the oesophagus : an anterior, which is developed into a 

 gland with a ciliated efferent duct, and a postei-ior, the 

 fundament of the pharynx (Fig. 158 F, dr, and s). Stout 

 cilia make their appearance in the circumference of the 

 body behind the mouth-opening, and form the post-oral cili- 

 ated band (Fig. 158 F). The embryo is now ready to hatch. 

 It has up to the present retained its spherical shape; but at 

 the time of hatching it passes into the permanent shape of 

 the larva, owing to the appearance of a constriction behind 

 the ridge that bears the circle of cilia (Fig. 159) ; this marks 

 off the broad anterior part of the body from the conical pos- 

 terior portion. At the same time the entire body enlarges, 

 and its cellular walls consequently become thinner. Hatch- 

 ing takes place by the pointed end of the lai^va breaking 

 through the serosa and egg-membrane at the pole opposite 

 the apical plate and the emergence of the embryo at this 

 point (Fig. 158 F). The connection with the serosa, as far 

 as it still exists, breaks, and the tuft of cilia of the apical 

 plate is withdrawn through the pores of the egg-membrane, 

 to be retained by the larva. The egg-membrane itself re- 

 mains for a while on the larva like a helmet. 



The larva of Sipunculus strongly resembles the Trocho- 

 phore, but differs from it in the absence of the preoral 

 ciliated band and the great reduction of the preoral part 

 of the prostomium (Fig. 159). As a result of this, the apical 

 plate comes to lie in the vicinity of the mouth, which is 

 shifted well toward the anterior end of the larva. The usual 

 three regions of the intestine can be recognized, though the 

 hind-gut opens to the exterior on the dorsal side (Fig. 159) ; 

 this, however, is frequently observed in Annelid larvffi. A 

 head kidney has not as yet been observed. The internal 

 organization is of a higher grade than is general in the 

 Trochophore, and in part already corresponds to that of 



