BRITISH ZOOPHYTES. 193 



depressions, showing the places where the eight-rayed 

 polyps have withdrawn. 



The spicules of the Alcyonaria ai'e favourite objects 

 with all microscopists. They occur imbedded in the 

 fleshy coenenchyma, and also in the body wall of the 

 polyps. They are calcareous, and generally assume 

 ragged and intricate forms. They are useful in afford- 

 ing support and giving compactness to the fleshy 

 structure. The polyps are inter-coo nected by a series 

 of canals which run through the entire system, and 

 are themselves connected with each other by smsiler 

 tributary and capillary passages. By this means the 

 zoophyte is nourished throughout, and is generally 

 kept distended with water. The Gorgnnidce differ from 

 the Ale.yonidce in having an internal flexible horny axis, 

 which is arranged generally in a palmate or fan-like 

 manner, as shown in Plate XV., fig. 5. This axis is 

 covered all over with a fleshy part in which the polyps 

 are placed. The fleshy part is strengthened by calca- 

 reous spicules. The horny axis also contains carbonate 

 of lime. The polyps project from the casnenchyma 

 like small warts, each wart having a star-like depres- 

 sion at the top, through which the polyp is exserted. 

 The axis varies in thickness according to age. It is 

 found in concentric layers, a transverse section having 

 an appearance under the lens not unlike that of an 

 exogenous stem. 



The remaining families are united into one sub-order, 

 the Pennatulida, or " Sea Pens,^^ the latter being a very 

 appropriate and significant name. Their structure and 

 life-history have received very great attention, Herr 

 Kolliker having written several works upon the subject, 

 and Messrs. Marshall having, in the report above 



o 



