138 THE ANATOMY OF INVERTEB RATED ANIMALS. 



of the umbrella ; under these circumstances the canals of the 

 umbrella in the Hydrozoon would answer to the intermesen- 

 teric chambers in the Actinozoon. 



Secondly, in the Actmozoa, the reproductive elements 

 are developed in the walls of the chambers or canals of the en- 

 terocoele, just as they so commonly are in the walls of the 

 gastro-vascular canals of the Hydrozoa, but the generative 

 organs thus constituted do not project outwardly, nor dis- 

 charge their contents directly outward. On the contrary, the 

 ova and spermatozoa are shed into the enteroccele, and event- 

 ually make their way out by the mouth. In this respect, 

 again, the Actinozoon is comparable to a Lucernaria modi- 

 fied by the union of the hydranth with the ventral face of the 

 umbrella ; under which circumstances the reproductive ele- 

 ments, which in all Hydrozoa are developed, either in the 

 walls of the hvdranth or in those of the oral face of the um- 

 brella, would be precluded from making their exit by any 

 other route than through the gastro-vascular canals and the 

 mouth. 



In the fundamental composition of the body of an ecto- 

 derm and endoderm, with a more or less largely developed 

 mesoderm, and in the abundance of thread-ceils, the Actino- 

 zoa agree with the Hydrozoa. 



In most of the Actinozoa, the simple polyp, into which 

 the embryo is converted, gives rise by budding to many 

 zooids which form a coherent whole, termed by Lacaze-Du- 

 thiers a zoanthodeme. 



The Coraxligexa. — The Actinozoa comprehend two 

 groups — the Coralligeiia and the Ctenophora — which are 

 widely different in appearance though fundamentally similar 

 in structure. In the former, the mouth is always surrounded 

 by one or more circlets of tentacles, which may be slender 

 and conical, or short, broad, and fimbriated. The mouth is 

 usuallv elongated in one direction, and, at the extremities of 

 the long diameter, presents folds which are continued into 

 the gastric cavity. The arrangement of the parts of the body 

 is therefore not so completely radiate as it appears to be. 

 The enteroccele is divided into six, eijrht, or more wide inter- 

 mesenteric chambers, which communicate with the cavities of 

 the tentacles, and sometimes directly with the exterior, by 

 apertures in the parietes of the body. The mesenteries which 

 separate these wide chambers are thin and membranous. Two 

 of them, at opposite ends of a transverse diameter of the Ac- 



