Characteristics of Hydrozoa. cHx 



The embryos are discharged by the mouth of the parent as free 

 ciliated larvae of smaller size and with fewer tentacula, or without 

 any^ but in other points they are like their parents. Reproduction 

 may take place also in the asexual ways of g-emmation and of fission, 

 and the entire animal may be regenerated from a separated frag- 

 ment ; but there are no ' Medusae' in this Class. 



Class, Hydrozoa. 



Coelenterata, which may be fixed or free, social or solitary, but 

 which are often of much smaller size, and invariably of a simpler 

 construction than either of the other two Classes in this Sub- 

 kingdom. The wall of the digestive cavity is continued directly 

 into that of the body cavity, or, in other words, the stomach is 

 never suspended freely in a perigastric cavity. The general body 

 cavity is prolonged, under the form of ' gastro-vascular canals,* 

 through the parenchyma of the body, and into the interior of the 

 tentacles. The body wall is made up of two layers, an ectoderm, 

 which in early stages is ciliated, and in later is very richly provided 

 with thread-cells; and an endoderm, which is ordinarily ciliated, and 

 keeps up a circulation in the g'astro-vascular fluids, and contains 

 also some, but fewer thread-cells than the ectoderm. The outer 

 layer of the ectoderm secretes a chitinous tubular polypary in the 

 fixed orders, with the exception of the Ilydrldae ; the deeper layers 

 of the body may attain in the free forms a greater or less amount 

 of induration, either in a disc-shaped (Medusae), or tube-shaped 

 [Siphonophorae) mass ; but it is only rarely that in the fixed orders 

 any calcareous deposit takes place, as in the LWiT/drodea. (See, 

 however, Kolliker, Icones Histiologicae, ii,, p. 117). No central 

 nerve-system has been demonstrated in these creatures; the mar- 

 ginally-placed cysticles of the Medusae may represent special sense 

 organs. The Hydrozoa are rarely hermaphrodite ; the generative 

 organs are developed in them as in all Coelenterata between the 

 two layers of the body walls, but owing to the absence of any 

 perigastric cavity, such as that of the Anthozoa and Ctenophora, 

 they come to be placed externally, and to be discharged into the 

 water not through the mouth, but simply by dehiscence of the 

 exterior layer. E/cproduction may be asexual in the way of gem- 

 mation frequently, and of fission rarely ; development may bo nearly 

 direct, as in Tulidaria and Coryne Van Denedenli, where the stage 



