246 On the Cape Cephalodiscus. 



a large anterior cavity and two pairs of clearly marked 

 coelomic sacs, representing the body-cavities of the collar and 

 raetasome respectively. These cavities are, perhaps, deve- 

 loped as enterocoeles. 



(vi.) A large area of the ventral ectoderm, which belongs 

 in the main to the anterior half, but generally extends con- 

 siderably beyond the middle of the larva, is much thickened, 

 and is occupied by numerous gland-cells resembling those of 

 the anterior or " ventral " wall of the adult proboscis or buccal 

 shield, and probably gives rise to that part of the zooid. 



(vii.) The anterior pole of the larva is provided with a mass 

 of clear vacuolated cells, situated in the ectoderm. This 

 organ is, perhaps, of sensory nature, and in G. indicus it has 

 been shown by Schepotieff (7) to bear a central tuft of long- 

 cilia and to be surrounded by a circlet of smaller cilia. 



(viii.) With the exception of the cilia just indicated the 

 larva is not known to carry definite rings of cilia, although 

 part at least of its ectoderm is uniformly covered with short 

 cilia. 



(ix.) The posterior pole of the larva is generally provided 

 with a definite ectodermic invagination of unknown nature. 

 This has been described under the name of the " posterior 

 pit" (2). 



(x.) The deeper parts of the ectoderm of the larva already 

 show signs of the development of a diffuse nerve-plexus. 



It appears improbable that there is any pelagic stage, and 

 the observations of Andersson seem to indicate that the larva 

 becomes a zooid by direct metamorphosis. There is at 

 present no evidence to show how the later stages of this 

 change are accomplished, and observations on this subject 

 and on the commencement of the formation of the ccencecial 

 tubes are specially required. 



Additional References to Memoirs on Cephalodiscus. 



(5) Andersson, K. A. " Eine Wiederentdeckung von Cephalodiscus. 1 ' 



Zool. Anz. xxvi. 1903, p. 368. 



(6) . "Die Pterobianchier der schwedisehen Siidpolarexpedition 



1901-1903." Wiss. Ergebn. schwed. Sudpolar-Exp. v. 1907. 



(7) Schepotieff, A. "Die Pterobranehier des Indischen Ozeans." 



Zool. Jahrb. Abt. Syst. xxviii. 1909, p. 429. 



(8) Braem, F. "Pterobiancbier und Brvozoen." Zool. Anz. xxxviii. 



1911, p. 546. 



(9) IIarmer, S. F., and Ridewood, W. G. "The Pterobranchia of the 



Scottish National Expedition (1902 to 1904)." Trans. Roy. Soc. 

 Edinb. xlix. 1913, p. 531. 



EXPLANATION OF PLATE XL 



Larva. X 295, of Cephalodiscvs yiichristi, drawn from a living - specimen. 

 The anterior end of the larva is at the upper part of the Plate. 



