W. G. RIDEWOOD. 



INTRODUCTION. 



THE genus Cephalodiscus was founded upon material dredged by the ' Challenger ' 

 in 1876 from a single locality, in the Straits of Magellan, Station 311. Since the 

 publication of the full report upon that species (C. dodecalophus) in 1887 by Professor 

 M' In tosh and Dr. S. F. Harmer (19), many papers have been written upon its 

 anatomy, mode of budding, and its systematic position, notably by Cole (2), Delage 

 and Herouard (3), Ehlers (4), Fowler (5), Harmer (7, 8, 9), Kemna (11), Lang (12), 

 Masterman (20-28), Schepotieff (29), and Spengel (31). No special methods were 

 adopted for the preservation of the material, so that the minuteness with which the 

 various investigators have succeeded in elucidating the histological details of the 

 polypides is not a little remarkable. 



In 1903 (1) Andersson notified the re-discovery of Cephalodiscus in the region of 

 the Falkland Islands by the Swedish Antarctic Expedition, but until the full report 

 is issued it is not possible to say whether this is a new species or not. 



In July of the past year (1905) was published the report on the Pterobranchia 

 of the 'Siboga' Expedition (10) by Dr. S. F. Harmer, containing a description of two 

 new species of Cephalodiscus obtained on the ' Siboga ' Expedition ( C. gracilis and 

 C. sibogae) and a third species obtained from the South end of the Corea Strait 

 ( C. levin seni). 



The only known polypides of C. dodecalophus are females, the colony of this 

 species being apparently dioecious ; the known specimens of C. gracilis are all females, 

 those of C. sibogae are males and neuters, and those of C. levinseni females. 



The material obtained by the ' Discovery ' consists of two species, the polypides 

 of both of which have an ovary and a testis, or two ovaries, or two testes. The one 

 species is bulky, and with large, deeply pigmented polypides, and the other has a 

 tubarium with long spine-like processes, like that of C. dodecalophus, but more massive, 

 and with the central cavity less divided up. The former species was named 

 C. nigrescens in a short account published by Lankester in the Proceedings of the 

 Royal Society of last year (15). The second species I now name C. hodgsoni, after 

 Mr. T. V. Hodgson, the biologist of the ' Discovery ' Expedition. Eight specimens 

 (pieces of colony) of this were obtained, five on one day, one two days previously, 

 the seventh four months later, and the last a fortnight later still. The dates and 

 localities are given on p. 49. 



I take this opportunity of offering my thanks to Professor Ray Lankester for 

 having entrusted to me the drawing up of the Report on Cephalodiscus, and I wish 

 further to express my indebtedness to him for frequent and valuable suggestions and 

 advice given during the progress of the investigation. 



