434 



THE ENTEROPNEUSTA. 



cavity or cavities, which may now be regarded as the definitive coelomic cavities 

 of the procephalon. (Figs. 296 and 297.) 



The Mesoderm and Ccelom.It is clear that the primitive proboscis ccelom, 

 or cephalic vesicle, is not comparable with the paired coelomic vesicles of the 

 collar and trunk, for the cephalic vesicle arises at a very much earlier period; it 

 consists of a special type of rounded cells not seen elsewhere, and from them arise 

 the first mesenchyme. It is unpaired, located on the neural side of the head and 

 widely separated from the coelomic vesicles of the trunk. The latter are paired, 

 have a posterior lateral position, are thin walled, and arise either as evaginations, 

 or as solid outgrowths of the teloccele wall, or as segments of mesodermic bands 

 of teloblastic origin. 



The cells forming the primitive proboscis vesicle (Fig. 270, B.), are comparable 

 with those that in arachnids (Fig. 141, ac.) arise from the inward proliferation of 

 the anterior primitive cumulus, or with those formed in the region of the cephalic 



FIG. 296. Diagrams to illustrate the larval development and metamorphosis of Balanoglossus. 



lobes in insects. In both cases, and this condition prevails no doubt in many 

 other arthropods, two distinct infoldings or solid ingrowths are formed, the anterior 

 or cephalic one representing the gastrula, the posterior, the teloblasts or teloccele. 

 (Fig. 269.) The anterior one, g, gives rise to a mass of cells at the point where 

 the stomodaeum is forming, or where it will appear later. From them arises the 

 mesoderm of the procephalic lobes, and the endoderm which either forms the 

 anterior portion of the enteron, or is scattered through the yolk and degenerates. 

 The Larva. The young larva, at about the time of hatching, is covered with 

 cilia and has somewhat the appearance of that in Fig. 296, B. It may now be 

 compared with an echinoderm larva, or with a legless, ciliated nauplius. (Fig. 

 296, A.) The main longitudinal, ciliated band represents, as it does in the echino- 

 derms, the free margin of the thoracic fold, and the margin of the caudal and pre- 

 oral lobes. The typical tornaria arises as a result of the contraction and subse- 

 quent infolding of the anterior haemal surface of the head. We have studied this 

 process and the part it play sin the formation of the cephalic navel of arachnids, and 

 in the formation of the mouth and the closing up of the haemal surface of the head 

 vertebrates. Chapter XIV, p. 253. Here we may again recognize the same process. 

 The cephalic navel first appears as a thickened depression of the haemal surface, B. 



