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CEPHALODISCUS 



By C. C. John, M.A. 

 (Plates XXXIII-XXXVIII, text-figs. 1-7) 



INTRODUCTION 



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

 in 1876 from the Straits of Magellan. It was first thought to be a compound 

 Ascidian and later referred to the Polyzoa. In 1887 its structure was satisfactorily 

 worked out by M'Intosh and Harmer (1887) who assigned it to the phylum Entero- 

 pneusta. 



Although C. dodecaloplms is the first recorded species, Ridewood (1921) has shown 

 that thirteen pieces of C. densm were dredged by the ' Challenger ' in 1874 at Kerguelen 

 Island. These were at first supposed to be Monaxonid sponges and later regarded as 

 a kind of hydrozoan allied to Spongicola fistidaris . They were correctly identified only 

 in 1919 and described by Ridewood in 1921 . Material of C. nigrescens had been obtained 

 as far back as 1841 or 1842 by the 'Erebus' and 'Terror', but remained unidentified 

 until Ridewood described it in 19 12. 



Under these circumstances C. dodecaloplms remained the sole representative of the 

 genus until 1903, when Andersson (1907) announced the rediscovery of Cephalodiscus 

 by the Swedish South-Polar Expedition. The Siboga report published by Harmer in 

 1905 added three new species. In the meantime Masterman and Schepotieff devoted 

 several papers to the further description of the Challenger material. The latter also 

 described an interesting new species, C. indiciis, from Ceylon. 



Lankester (1905) described a new species, C. nigrescens^ from the Discovery material 

 of the 1901-1904 Expedition. In 1906 Ridewood described C. gilchristi from South 

 Africa and in 1907 gave an account (1907 b) of the two species which had been dredged 

 by the Discovery Expedition of 1901-1904. Among the collections of the " Scottish 

 National Antarctic Expedition" there were some colonies of C. nigrescens, C. hodgsoni, 

 C. solidus and C. agglutinans which were described by Harmer and Ridewood (1913). 



Finally when Ridewood (1918 rt) published his reports on Cephalodiscus of the Terra 

 Nova Expedition, he gave a complete synopsis of all the species then known. Before 

 that time it was found necessary to break up the genus into three sub-genera. To these 

 I have found it necessary to add one more sub-genus Acoelothecia to accommodate one 

 of the new species, C. kempi, of the present collection. 



Up to now, sixteen species have been described, including those which are regarded 

 as doubtful forms, and the present Expedition adds two more new species to the list. 



The material of Cephalodiscus collected during the Discovery Investigations occupies 

 twenty-two jars. Although large quantities were collected from some stations, the yield 



