ENTEROPNEUSTA FROM THE ABROLHOS ISLANDS. 95 



fibres (PL 11. fig. 9) which have been noticed by previous observers in other 

 species o£ Enteropneusta occur at the posterior end of the organ. 



Nervous System. 



~As is well known the nervous system o£ the Enteropneusta consists of a 

 nerve-fibre Layer present in relation with ectodermal epithelium. In certain 

 places this layer is very raueh thickened, giving rise to so-called nerve-trunks 

 or nerve-cords. The general structure of the nervous system o£ Pt. pelsarti 

 agrees with that of described species, and so reference will only be made here 

 to points of systematic importance. 



The most important nerve-trunk . is the Collar Nerve-Cord. This cord, in 

 the genns Pti/chodera, is frequently a true medullary tube (Pt.JIava) pos- 

 sessing a central canal opening to the exterior at both anterior and posterior 

 ends. In some cases only a few cavities are to be seen [Glossobalanus 

 minuta, Balanoglossus australiensis, etc.). In the species now being described 

 there is a well-developed continuous canal (PI. 10. fig. 2, Can.N.C.) with 

 anterior and posterior neuropores (fig. 2, Ant.N.P. and Post.N.P.). This 

 appears to be characteristic of all members of the genus Ptycliodera^ 

 although Punnet found a marked tendency to occlusion of the canal in many 

 specimens of his varieties of Pt. fiava. There is no tendency towards 

 occlusion in any of the specimens of Pt. pelsarti examined. 



In transverse section the cord is everywhere much broader than deep 

 (dorso-ventral measurement) (PI. 11. fig. 10). 



The nerve-substance is almost confined to the ventral side of the tube 

 (fig. 2, D.N.C.). It is much more thin laterally, and dorsally can only just 

 be recognised. In fact_, the dorsal and ventral walls of the collar nerve-cord 

 iire entirely different. The cells of the dorsal wall are very delicate, and not 

 nearly so numerous as the cells of the ventral wall. The ventral wall agrees 

 much more with the external epithelial layer of the proboscis, particularly 

 with that of the anterior neuropore region. It is true that gland-cells are 

 not so numerous, but they are present in very considerable numbers, and 

 quantities of mucus may be found in the lumen of the nerve-trunk. There 

 are very few gland-cells amidst the cells of the dorsal wall, except in the 

 posterior region of the collar, where they become slightly more numerous. 

 There are no traces of giant ganglion cells. 



As in other species of Ptycltodera, the collar nerve-trunk is united with the 

 epidermis at intervals by unpaired dorsal roots (fig. 2, P.P.', D.R."). These 

 are few in number, three roots being most common. The anterior root arises 

 in front of the middle point of the cord, almost in the same plane as the 

 opening of the stomochord into the throat-cavity (fig. 2). This agi-ees with 

 Hill's description of Balanoglossus australiensis, and is contrary to Spengel. 

 The roots arise as hollow diverticula from the medullary tube and run 



