ENTEROPNEUSTA FROM THE ABROLHOS ISLANDS. 93 



the basement-membrane, which underlies the small-celled epithelium o£ the 

 so-called racemose organ. 



A horizontal section taken through the neck of the proboscis in the region 

 just described (PI. 11. fig. 8) shows some of the characteristic features of 

 the species. 



Posterior to the region described above, the keel separates from the 

 ventral proboscis csecum, and at the same time loses its concavity and the 

 clasping ventral margins (fig. 7). The lateral edges run out into chondroid 

 tissue. Passing still further in a posterior direction, we 6nd that the 

 skeleton becomes more and more elongated in a lateral direction, still 

 keeping a ventral keel, until finally this disappears and the transversely 

 elongated skeleton diverges into the two posterior limbs — the cornua. 



The cornua commence rapidly to diverge and embrace the oesophagus. 

 Now it is characteristic of the Ptychoderidse that the cornua of the nuchal 

 skeleton terminate in front of the middle region of the collar, and in most 

 species they do not pass back very far. In Punnett's varieties of Ptycliodera 

 fiava the cornua only extend one-seventh to one-fourth the length of the 

 collar, except in one variety, Pt. flava, var. cooperi, Punnett^ where they 

 attain such an extraordinary length that they approximate the posterior end 

 of the collar. In Pt. pelsarti the cornua (fig. 10, Sk.') extend more than 

 halfway down the length of the collar, and embrace the oesophagus to such 

 an extent that near their terminations they are much nearer the mid-ventral 

 line than the dorsal surface (fig. 10). This is quite a characteristic feature 

 of the species. Apart from the great length of the cornua in Punnett's 

 variety, there is but little resemblance between that form and Pt. pelsarti. 



Chondroid Tissue of the Nuchal Skeleton. 



The cartilage-like chondroid tissue, referred to at great length by Spengel 

 in his well-known monograph, is well developed in the present species. It 

 is impossible, however, to my mind, to speak of it apart from the skeleton — 

 or to figure the skeleton as an isolated structure, as is sometimes attempted. 

 There is no definite line of demarcation, in the best sections, between the 

 chondroid tissue and the substance of the main mass of the nuchal skeleton 

 (fig. 8, Sk. 8f Ch.T.). 



Furthermore, the skeletal substance passes quite insensibly into the base- 

 ment-membrane, and it is evident that in the nuchal skeleton we have a 

 structure which is a local development of a tissue of wider extent. The fact 

 that in some species of Enteropneusta, i. e. Balanoglossus clavigerus, there 

 are numerous small cells scattered throughout the skeleton still further 

 emphasises the point that the chondroid tissue is merely an extension of the 

 substance of the skeleton. 



In a transverse section, taken at about the plane of the proboscis pores, 

 the chondroid tissue extends almost completely round the central blood-sinus 



