THE ANATOMY OF ALOYONIUM DIGTTATUM. 355 



projections for a long distance down the coelentera of the fused 

 portion of the polyps, and the endoderm shows no important 

 modification in that region. Consequently the coelenteron of 

 the fused portion of each polyp is no more "colonial" in 

 character than that of the free extensible portion. It is, in 

 fact, simply a part of the individual polyp. 



A comparison with the genus Tubipora may render this 

 matter clear. In this genus the polyps remain separate from 

 one another for the greater part of their course, being con- 

 nected together only by the horizontal platforms and the basal 

 stolon. The only part of the colony that is, strictly speaking, 

 " colonial," or, in other words, common to more than one 

 polyp, are the stolon at the base and the horizontal platforms. 

 In these structures there runs a number of branching canals, 

 lined by endoderm, which establish a communication between 

 the polyps, and form a basis from which new polyps originate 

 as buds. In the case of the colonial Hydrozoa it is often a 

 matter of difficulty to determine where the coelenteron of a 

 polyp ends and the coenosarcal canal begins, and consequently 

 the distinction between polyp and ccenosarc is indefinite. But 

 in the case of the Alcyonarian there is no such difficulty. The 

 coelenteron of a polyp of Tubipora, for example, can be distin- 

 guished from a canal in a horizontal platform at once by the 

 fact that it contains, either complete or in a shrivelled condi- 

 tion, the dorsal mesenteries and mesenterial filaments. 



In Tubipora, then, there can be no doubt that the ccenosarc 

 is composed of the horizontal platforms and basal stolon, and 

 by these structures only. 



In the case of Alcyonium the distinction between the polyps 

 and ccenosarc cannot be defined. In consequence of the enor- 

 mous development of the mesogloea in this genus, the parts 

 corresponding to the polyp walls and horizontal platforms and 

 basal stolon of Tubipora are all fused together, and conse- 

 quently it is impossible to say where the polyp wall ends and 

 ccenosarc begins. 



The colony, then, is mainly composed of a number of polyps 

 partially fused together. Some of these — the primary polyps 



