PHYSIOLOGY OF THE INVERTEBRATA. 33 



dilated portion, and a narrowed portion, which ultimately 

 terminates in a caecum. 



The Hijdrozoa which live in colonies, have an intestinal 

 tube in common — that is, the tube acts for the whole tribe of 

 organisms ; while in the Siphonophova (one of the four orders 

 of the Hydrozoa) " certain members of the colony specially 

 adapt themselves for the digestive function. For that pur- 

 pose they come to bear the form of dilatable sacs, and are 

 in communication interiorly with the digestive cavity common 

 to all the tribe." 



In the Adinozoa the body cavity is hardly more differen- 

 tiated than that of the Hydrozoa, for it is still a gastro-vascular 

 cavity, but is divided by vertical partitions (mesenteries) into 

 a number of interraesenteric chambers which communicate 

 with each other at the bottom of the gastro-vascular cavity. 

 The mouth or oral aperture of the Actinozoa serves both for 

 the reception of food and the rejection of excreta. 



The mesenteries of the Adinozoa are divided into primary 

 and secondarv ; the former being longer than the latter. 

 The secondary mesenteries, which are situated between the 

 primary, have on their free edges twisted and coiled filaments. 

 These mesenteric filaments secrete a fluid which has digestive 

 properties. 



The Adinozoa may be divided into two groups — the 

 Corcdligcna and the Ctcnophora. Most of the Coralligerca 

 " are fixed temporarily or jaermanently, and may give rise to 

 (by gemmation) tuft-like or arborescent zoanthodemes. The 

 great majority possess a hard skeleton, composed principally 

 of carbonate of lime." This group of the CuBlcnterata has a 

 digestive or somatic cavity divided by mesenteries. 



The Ctcnophora are free swimming organisms, and never 

 give rise to colonies. They possess a mouth or oral aperture, 

 an oesophagus, and a gastro-vascular canal system, which 

 communicates with the exterior by two aboral apertures. 



So far we have seen, in this general study of the digestive 

 function of the Invcrtchrata^ this function performed by the 



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