224 CXELENTEEATA. 



of the Hydromedusse by their larger size and the more com- 

 plicated structure of their gastrovascular cavity (fig. 43). The 

 latter is not a simple cavity in the body, but is divided by numerous 

 vertical partitions, the mesenteric folds, into a system of vertical 

 pouches which communicate with one another at the bottom of 

 the gastric cavity. In addition a system of capillary passages is 

 also frequently present in the body wall. At their upper extremity 

 the pouches are continuous with the canals leading into the hollow 

 tentacles, since the edges of the mesenteries bounding them unite 

 with the wall of the oral tube which hangs from the mouth. An 

 opening may however persist in each mesentery underneath the oral 

 disc, putting the neighbouring chambers in communication. The 

 oral tube has the significance of an oesophagus, and possesses at its 

 internal end, where the peripheral chambers open into the central 

 cavity, an opening capable of being closed, by means of which its 

 cavity stands in communication with the gastrovascular system. The 

 mouth is used not only for the reception of food, but also for the re- 

 jection of excreta. The secretions of the coiled and twisted filaments 

 (mesenteric filaments) at the edge of the mesenteries must be regarded 

 as aiding in digestion (fig. 43). 



The body of the polyp consists of an external coating of cells, 

 an internal layer lining the gastric cavity, and an interposed 

 connective tissue layer of very various thickness and structure 

 (mesoderrn). The latter appears rarely as gelatinous tissue, and 

 more frequently as a tough homogeneous connective tissue containing 

 spindle and star-shaped cells (Alcyonidce, Gorgonidce). This tissue can 

 also assume the form of fibrous connective tissue, and become the 

 seat of calcareous deposits. Muscle fibres, which take their origin 

 from the entoderm cells, can also appear in the mesoderm ; while the 

 newly discovered ectoderrnal sense epithelium and nerve fibrillse 

 keep their superficial position in the region of the oral disc and on 

 the tentacles. The generative products arise on tne mesenteries 

 near the mesenteric filaments as band-shaped or folded thickenings, 

 and, according to Hertwig, are products of the entoderm. The sexes 

 are for the most part separate, although hermaphrodite individuals 



u The Structure and Distribution of Coral Reefs," London, 1842. J. D. Dana, 

 " United States Expl. Expedition. Zoophytes." Philadelphia. 1846. M. Edwards 

 et J. Haime, " Histoire naturelle des Gorailliares," 8 Tom, Paris, 1857 1860. 

 Laeaze Duthiers. ' Histoire naturelle du Corail," Paris, 1864. Gosse, " Actino- 

 logia britannica.'' London, 18fiU. Kiilliker, ' Anatomiseh-systematische Beschrei- 

 buii? der Alcyonaricn,'' 1872. Moseley, " The Structure and Relations of the 

 Alcyonarian Heliopora ccerulua. etc." PhihixupJi. Transm-timm oftJie Roy. Sec., 

 1876. 



