PYROSOMA- THE FOUR PRIMARY ASIDIOZOOIDS. 409 



cavity. The rudimenl of the stomach and intestine is found on the 

 lower surface of the entoderm-tube in the form of a blind horseshoe- 

 shaped diverticulum (Fig. 200 B) closely bent upon itself, the free 

 ends being directed anteriorly. The two limbs of the horseshoe are 

 seen cut through in the cross-section given in Fig. 204 (oe and ed). 

 One limb (ed) of this rudiment separates early from the entoderm-sac 

 and, as a blind diverticulum, represents the rudiment of the intestine, 

 while the other gives rise to the stomach and oesophagus. The com- 

 munication between this latter limb and the pharyngeal cavity is re- 

 tained as the entrance to the oesophagus (oe). Only in later stages 

 does the blind end of the intestine become connected with the atrial 

 cavity and the alimentary canal attains complete development with 

 the appearance of the so-called digestive gland. 



ms 



Fig. 202. — Transverse section through the anterior region of an Ascidiozooid' of 

 Pyrosoma with the rudiment of the nervous system (after Salenskt). ec, ectoderm ; 

 i-ii, entoderm; ms, mesoderm-cells ; n, rudiment of the nervous system ; p, peri- 

 branchial tube. 



The central nervous system arises in the most anterior region of 

 the Ascidiozooid as an ectodermal invagination (Fig. 202, n) on the 

 upper surface of the body, which soon becomes separated from the 

 ectoderm as an elongate closed vesicle. This, at a later stage, becomes 

 triangular (Fig. 200, n). The anterior part* of this vesicle forms the 

 rudiment of the ganglion proper principally by a growth of its upper 

 wall. From this region, two lateral hollow processes arise (Fig 200. 

 m) which, at a later period, grow and embrace the sides of the ali- 

 mentary canal. These are the rudiments of the lateral nerves which 

 thus arise here in a way similar to that described by Seeliger for 

 the later buds of Pyrosoma. The narrowed portion of the neural 

 rudiment which is directed towards the branchial aperture becomes 

 connected with the ectoderm of the alimentary canal, and, by a per- 



* [Anterior in relation to the long axis of the bud. poste/fo^B^ibM^scl&iltyV 



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