CHAPTER XTII. 



ENTEROPNEUSTA. 



Under the name of Enteropneusta it is customary to place 

 the isolated form Balanoglosstis next to the Echinodermata, 

 since it scarcely presents closer relationship to any othei- 

 division. At the close of this chapter something further 

 will be said on its probable position in the system. In order 

 to make ourselves more easily understood concei-ning the 

 developmental processes, it seems necessary to discuss first 

 some morphological conditions.^ 



Anatomical. — Balanoglossus possesses anelongated vermi- 

 form body, on which different regions can be recognized 

 externally. Anteriorly the so-called acorn [balanus], less ap- 

 propriately called proboscis, is marked off from the rest of 

 the body ; upon this follows the muscular collar, and then the 

 branchial region, which gradually merges into the posterior 

 part of the body (Fig. 164). Acorn and collar areessentially 

 a locomotor apparatus, and therefore are largely composed of 

 muscle fibres, which can be distinguished as external circular 

 and internal longitudinal muscles. The cavities inside both 

 organs which are left between the longitudinal muscles and 

 connective-tissue cells can be filled with water from the out- 

 side by means of one or two dorsal pores lying at the base 

 of the acorn (Fig. 165 p). Similar pores also conduct water 

 into the cavities in the collar (Spengel). These conditions 

 have been compared to those of the water-vascalar system of 



1 [It should be mentioned here that since the publication of our de- 

 scription of the development of Balanoglossus the important works of 

 Spengel (No. VI.) and Morgan (Nos. II., IV.) have appeared, necessitating 

 some modifications in the account which we have given. The most im- 

 portant of these will be pointed out in what follows. — K.] 



373 



