194 DISCOVERY REPORTS 



The material for the present paper was obtained in an unexpected manner from the 

 collections of the Terra Nova Expedition, which are now preserved in the Natural 

 History Museum. When I was engaged in reporting on the Cephalodiscus of the Dis- 

 covery Expedition I was kindly permitted by Miss Hastings to examine the Terra Nova 

 collections. On one occasion I was prompted to collect some of the sediment which had 

 settled at the bottom of jars containing colonies of C. tiigresce?is. I examined this in 

 October 1932 and discovered twenty-one larvae in different stages of development, and 

 two embryos. These I showed to Prof. E. W. MacBride, F.R.S., and at his kind 

 suggestion this work was undertaken. 



As there were only a very limited number of specimens, whole mounts were not made, 

 and though every precaution was taken in preparing the sections the results were not 

 quite satisfactory, owing to the fact that the material was very old and imperfectly 

 preserved. The description of the genital organs and buds is based on Discovery material 

 of C. nigrescem. 



I take this opportunity to record my deepest gratitude to Prof. E. W. MacBride, 

 F.R.S., for his kind help and encouragement and the facilities he gave me for carrying 

 out this work in the Huxley Laboratory of the Imperial College of Science and 

 Technology. I am also indebted to Miss Hastings, who is in charge of the Prochordate 

 section in the Natural History Museum, for kindly permitting me to examine the Terra 

 Nova collections. 



GENERAL CONSIDERATIONS 



The zooids of Cephalodiscus are either unisexual or hermaphrodite. The colonies of 

 C. dodecolophus, C. levmseni and C. gracilis have been found to contain only female 

 zooids in the specimens so far obtained, while the colonies of C. nigrescens, C. densus, 

 and C. kempi are formed of male, female and hermaphrodite zooids. In these species 

 the three different kinds of zooids are not differentiated by external characters, so that 

 it is impossible to identify them except by sectioning the adult zooids. On the other 

 hand the two species C. hodgsoni and C. sibogae show striking instances of dimorphism. 

 In C. hodgsoni the female zooids are crimson-brown and possess twelve arms, while the 

 male zooids are pale brown and possess ten or eleven arms. Some colonies are formed of 

 both male and female zooids, while others are entirely of one sex. In C. sibogae the 

 ordinary zooids are usually completely sterile or neuter individuals, in which the repro- 

 ductive organs are vestigial. The male zooids are entirely different in appearance and are 

 found in the basal jelly of the colony. They possess only a single pair of arms without 

 pinnules. The alimentary canal is vestigial and there is no trace of a glandular stomach. 

 It is believed that these male zooids are nourished through the vascular system by the 

 neuter individuals. 



In the male zooids of C. hodgsoni, C. nigrescens and C. densus the testes are a pair of 

 organs symmetrically placed, one on either side of the median line, in the anterior part 

 of the trunk cavities between the pharynx and the anus. Each testis is a small ovoid body 

 with a narrow lumen in the centre. The testis therefore appears like a thick- walled 

 hollow sac, the lining epithelium of which gives rise to the spermatozoa. The lumen of 



