86 COMPARATIVE ANATOMY CHAP. 



a circumferential canal at the margin of the disc by means of breaches 

 through these septa. If the septa are reduced to 4 small points of 

 concrescence between the exumbrellar and subumbrellar walls of the 

 peripheral intestine, directly on the circumference of the main intestine, 

 the 4 pouches coalesce to form a spacious circular sinus, which com- 

 mands the whole edge of the disc, and communicates with the principal 

 gastric cavity between the points of concrescence. In the higher 

 Acraspeda this circumferential sinus becomes divided, by the partial 

 concrescence of its exumbrellar and subumbrellar walls, into 8, 16, 32, 

 or more radial chambers or radial canals, which in many forms, by 

 anastomosing or branching, form a very ornamental net- work of canals 

 running towards the edge of the disc (Fig. 70, 0). 



Excretory pores of the gastro-eanal system. In various Medusw, 

 in Acmspeda as well as in Craspedota, small apertures have been observed 

 at the margin of the disc ; these often lie on the points of papillae, and 

 by means of them communication between the peripheral gastro-canal 

 system and the outer world is established. 



The gastro-eanal system of the Ctenophora (Fig. 68, p. 79), in 

 its general arrangement, has already been delineated. We add here 

 that the meridional vessels in lobate Ctenophora, and also in the Cestidce 

 and Beroidce, communicate with each other and with the cesophageal 

 vessels at the oral portion of the body, and that in the Beroidce they 

 also send out numerous branching and anastomosing processes, some 

 of which enter the jelly, or join to make a peripheral net-work. The 

 nourishment of the often much -developed oral lobes of the lobate 

 GtenopJwra is provided for by the meridional vessels, which traverse the 

 oral lobes in various arabesque-like patterns. 



Histological. Each epithelial cell of the gastro-canal system very commonly 

 carries one single flagellum ; these cells are thus flagellate cells. Among the epithelial 

 cells there are gland cells, stinging cells, cells with various contents as products of 

 metabolism, epithelial muscle cells, etc. Veiy often the gastric epithelial cells send 

 out amceboid or pseudopodia-like processes on that side of them which is turned to 

 the lumen of the gastro-canal system, and by the help of these they take into their 

 cell bodies small particles of food in the manner of the Rhizopoda (intracellular 

 ingestion of food). 



IV. Musculature. 



In the Hydroida and Siphonoplivra we find, in the first place, a 

 system of longitudinal fibres which run, buried in the epithelium, from 

 the oral to the aboral pole, and in the tentacles. These fibres, which 

 correspond to the processes of the ectodermal neuro- muscular or 

 epithelial muscular cells, serve for contracting the body and the 

 tentacles. In these forms again, and especially in the Siplionopliora, 

 there is a system of circular fibres which run under the endodermal 

 epithelium as processes of the endodermal epithelial muscular cells. 

 By the contraction of these fibres the body and the tentacles are 



