594 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



In sections throuyh the upper regions of the polyp the continuity of the nicsentories is often 

 interrupted by the presence of synapticuhir ])erf orations; further, some of the mesenteries are 

 continuous from one polyp to another. The continuity of the mesenteries of contiguous polyps 

 is without doubt to be associated with the abs(Mice of distinct jiolyjial limitations noticed amono- 

 the external characters, and also with the confluent septo-cost;v characteristic of the genus; 

 probably also it hns some bearing upon the method of asexual growth of the colony, which calls 

 foi' moi'e detailed study. 



Mesenterial tilaments occur on all the mesenteries, but in the upper region are very 

 rudimentary in character, and imperfectly separated from the mesenterial epitheliinn. In the 

 lower region of the polyp many of the tilaments undergo an enormous develo]iment in connection 

 with the convolution of the mesenteries. They mostly All the septal loculi, and l)ear numerous, 

 closely arranged, large nematocysts, and many clear, brightly staining gland cells, and others 

 with coarsely granular contents. 



The skeletogenic ectoderm is i-arely determinable in ordinary decalcified material, but 

 desmoidal processes are lunneroiis along the line of attachment of some of the mesenteries. The 

 skeletotrophic endoderm reuuiins a very narrow layer throughout, undergoing but little increase 

 in thickn(>ss in the lower regions. 



In the ordinary condition of retraction the gastro-ccelomic cavity is very limited in extent. 

 The central cavity is prolonged upward and outward ])etween the mesenteries and the septa as 

 far as the edge of tiie theca, and is there placed in communication with that of adjac'cnt polyps 

 (fig. 162). Downward tlie cavity soon diminishes in peripheral extent, and centrally is lu'oken 

 up into distinct cham))ers by the inward growth of the septal invaginations which meet in tii(> 

 middle. For some tlistance the interseptal loculi are crowded with the enlarged and convoluted 

 mesenterial filaments. 



