INTERNAL ANATOMY 



249 



each side of the pharynx as a band of vacuolated cells with a deep internal groove com- 

 municating with the cavity of the pharynx. The insertion of these lateral bands, the 

 pleurochords, divides the wall of the pharynx more or less completely into four distinct 

 regions, two lateral, a mid-dorsal and a mid-ventral. The two lateral regions are formed 

 of the bands of vacuolated cells, and the mid-dorsal and mid-ventral walls of ordinary 

 cells of the pharynx. The groove on the inner side of the pleurochords is deep and 

 conspicuous in C. hodgsoni, and, as already mentioned, in C. densiis a tubular space com- 

 pletely lined with vacuolated cells lies between the external gill opening and a cavity of 

 the pharynx. This has been termed the gill cavity, and its opening into the pharynx the 

 internal gill slit. The respiratory water passes through the gill cavity before it finds 

 outlet through the external gill slit. This gill cavity could have been formed only by the 

 fusion of the lips of the groove in the pleurochords. In other species where the lips of the 

 groove stand apart, the respiratory water probably traverses the groove before passing 

 through the gill slit. Assuming this to be so, it is clear that the pleurochords and the 

 grooves play a definite part in respiration. Further, the pharyngeal blood vessels lead 

 to the vacuolated region of the roof of the gill slit, but the mode of exchange of gases 

 still remains a problem. 



A rod-shaped structure with a narrow lumen projects upwards from the dorsal wall of 

 the anterior diverticulum of the pharynx. 

 This organ has been variously termed by 

 diff'erent writers. Harmer (1887, p. 40) 

 compared it with the notochord of 

 Balanoglossiis . Later, Willey (1902) de- 

 scribed it as the stomatochord, and 

 Masterman (1898 h, p. 351) regarded it 

 as the subneural gland; but the view 

 most generally accepted was that this 

 organ corresponds to the vermiform pro- 

 cess of certain Enteropneusta. In this 

 paper it is called the notochord in accord- 

 ance with the view of Harmer. The 

 lumen of the notochord opens into the 

 dorsal diverticulum of the pharynx. 

 Masterman thought this the enlarged base of the duct, but the evidence is not strong 

 enough to support the glandular nature of the notochord: I did not find a trace of 

 mucus inside the lumen. 



Since the development of CephaJodiscus has not been worked out thoroughly it is 

 difficult to say anything definite about the origin of the notochord, but it may be worth 

 mentioning that during the development of the bud it is very conspicuous. In Fig. 6, 

 A and B, drawings are given of sagittal sections of the young bud. The proboscis is very 

 large and the collar cavities have just appeared as a ring. Within the trunk-cavity is the 

 rod-shaped primitive gut, which has not as yet mouth-opening or intestinal loop. Later, 



Fig. 6. Young bud of C. tiigrescens (sagittal sections). 

 A, early stage; B, later stage, a, proboscis; d, noto- 

 chord; y, heart and pericardium; /, collar cavity; 

 w, mouth; «, primitive gut. 



