82 COMPARATIVE ANATOMY CHAP. 



In the body epithelium of the Ctenophora we find peculiar adhesive 

 cells (Fig. 69, c) with uneven and sticky surfaces. Their bases are 

 prolonged into spirally coiled contractile filaments. 



Besides the nerve cells, the sensory, muscle, stinging, flagellate, 

 ciliated, and covering cells, various glandular cells and pigment cells 

 are found in the body epithelium of many Cnidaria, and are especially 

 numerous in the Ctenophom. 



III. The Gastro-eanal System. 



The gastro-canal system in its general arrangement has already 

 been mentioned in the general review. It is the most characteristic 

 system of organs of the Cnidaria, and in some groups reaches a very 

 high degree of complication. This complication stands in direct rela- 

 tion to the complication of other portions of the body, which fact is at 

 once comprehensible when we learn that in the Cnidaria the gastro- 

 canal system undertakes not only the digestion, but also the circulation. 

 The more massive the body and the more numerous and complicated 

 its organs, the more necessary is it that the nutrition of these organs 

 should be provided for by gastro-canals or vessels. 



From such a standpoint the complicated arrangement of the gastro-canal system 

 of the Medusae and Ctenophora is at once comprehensible. In the Medusce the 

 margin of the umbrella is distinguished by the possession of numerous organs 

 (tentacles, velum, auditory vesicles, stinging cells, nerve ring, eye spots, and sensory 

 bodies). The subumbrellar side is strongly muscular, while the exumbrella is devoid 

 of organs. The jelly being so largely developed as a passive organ for motion and 

 support, the special gastro-canals (radial vessels) must run near the subumbrella to 

 convey food to the organs on the margin of the disc. This purpose is also served by 

 the circumferential canal, into which they enter. The relations of the gastro-canals 

 to the other organs of the body are just as clear in the case of those Ctenophora 

 which have a massive gelatinous tissue between the ectoderm and endoderm. The 

 most important organs of the body, apart from the sexual organs, are in this group : 

 the sensory body of the aboral pole, the 8 ribs, and the 2 tentacles ; answering to 

 these, we find an aboral vessel, also 8 vessels running from the stomach to the ribs, 

 and which enter 8 meridional vessels, and, further, 2 vessels which run to the base 

 of the tentacles. 



According to the ontogenetic origin of the gastro-canal system, two 

 principal types can be distinguished in the Cnidaria. In one type, 

 which is found among the Hydrozoa, the whole gastro-canal system 

 rises out of the endodermal enteron of the larva. In the second type, 

 which is characteristic of the Scyphozoa and Ctenophora, the gastro- 

 canal system consists of an ectodermal and an endodermal portion. 

 The former, which we call stomodaeum in embryos, larvae, and 

 generally in young transition stages, and oesophagus in adult animals, 

 arises at the oral pole by a depression of the ectoderm into the body. 

 This oesophagus is represented in the Scyphomedusce by the inner 

 lining of the oral or gastric peduncle as far as the point of insertion of 



