GONADS OF CEPHALODISCUS NIGRESCENS. 45 



p. 412, footnote) compares with the chordoid structure which he found to exist 

 (22, p. 305) in the mid-ventral wall of the stomach of Actinotrocha. 



From all this it would appear to an unprejudiced mind that the gut-wall may 

 develope tracts of vacuolated skeletal cells in any part of its extent, and that, 

 except in the case of the notochord of the Cephalochorda and Vertebrata, which 

 occupies a definite position between the central nervous system and the enteric 

 tube, any close comparison of these various tracts in the different forms of the 

 Chordata is almost impossible. 



The suggestion of Masterman (22, p. 356, and 25, p. 915) that the pleuro- 

 chord of Cephalodiscus is a skeletal structure developed in the wall of the pharynx 

 for the purpose of keeping open the gill-cleft, and Willey's remark (34, p. 238) 

 that a long pleurochord may mark the position of a row of obliterated gill-slits, 

 are worthy of careful consideration. In the Cephalochorda, Cyclostomata, Fishes, 

 and Amphibia, the positions between the gill-slits are supported by .skeletal tissue 

 (mesoblastic, it is true, whereas the pleurochord is hypoblastic), and it is not 

 unreasonable to suppose that the long pleurochord of Cephalodiscus, with the 

 gill-slit at its anterior end, may represent the skeleton of a series of slits, only 

 one of which now appears in ontogeny ; and further, that in Actinotrocha 

 (Masterman, 22) and Rhahdopleura (Schepotiefi', Bergens Mus. Aarbog, 1904, p. 14, 

 Kiemenrinne) the skeletal matter of the slit develops ("liver dverticula" of earlier 

 writers (Masterman, 22, p. 304)), although the slits themselves never make their 

 appearance. Willey claims that the lateral pouches of the stomochord of the 

 Enteropneusta are the " persistent vestiges of primitive gill-clefts belonging to that 

 portion of the body which, in the Enteropneusta, is now specialised as the collar 

 region" (34, p. 238). 



Gonads. 



The individuals of the species under consideration possess either two ovaries or 

 two testes, or an ovary and a testis. In a series of thirty-six individuals examined, 

 fifteen had two ovaries, seven two testes, and fourteen an ovary and a testis. The 

 three kinds of individuals are not restricted in their distribution. The same branch 

 of the colony may have male, female, and hermaphrodite individuals, and no 

 distinction can be drawn as regards sex between the individuals found in the basal, 

 middle, and more terminal portions of the same branch of the colony. 



If the gonads are large they bulge somewhat upon the sides of the body, and 

 by stretching the pigmented body-wall that covers them, cause it to be paler than 

 the other parts of the surface (see figs. 7 and 8, plate 3). 



The ovary is narrow at its anterior extremity, where it opens upon the exterior 

 by a short duct with a small, frequently ill-defined cavity, and with walls of a red 

 colour, which resembles that of the red line in the buccal .shield in tint and in being 



