230 GOILENTERATA. 



may occur, the latter usually by budding, niore rarely by division. 

 Sexual and asexual types of reproduction can be combined in the 

 same species, producing an alternation of generations. 



In compjarison with the sp)onges the Coelenterata may be called 

 epithelial organisms. A mesoderm (' mesoglosa') may be entirely 

 lacking or may have but a subordinate develoi:)ment. The ectoderm 

 and entoderm, on the other hand, are the important tissues — pro- 

 ducing muscles, nerves, sense organs, sexual products and cnidaj. 

 Hence the group is often called Diploblastica — two-layered animals. 



Class I. Hydrozoa (Hydromedusae). 



According to varying standpoints the Hydrozoa can be j)laced 

 either higher or lower than the Anthozoa in the system, since in the 

 former group two forms are frecjuently introduced into the life 

 history, one agreeing well in structure with the Anthozoa, the 

 other standing on a higher grade. The first is the sessile and 

 usually colonial polyp, the second the Iree-swimming medusa, well 

 provided with sense organs. These are usually related to each 

 other by an alternation of generations. The polyp is asexual and 

 by budding produces medusfe; the medusa, on the other hand, is 

 the sexual stage, and from its eggs polyjjs arise. 



The polyp of the Hydrozoa is the hydropolyi^, forming in the 

 branch of coelenterates an important archetype from which all 

 other conditions — medusas, scyphopolyj), and even the coral polvp — 

 may be derived. Our best example of this is the fresh-water 

 Hydra, so common in pools and streams. The body (fig. 172) is a 

 sac, the hinder closed end of which, the pedal disc, is used for 

 attachment. The other end bears the mouth which leads to the 

 internal gastrovascular (digestive) cavity. Around the mouth is a 

 circle of tentacles used in capturing food (mostly small Crustacea). 

 These are outgrowths of the body wall; the circle dividing the 

 body into a peristome inside the circle and a column constituting 

 the rest of the outer wall. 



Hydra has but two body layers (fig. 173), an entoderm of 

 flagellate cells lining the gastrovascular space, and the ectoderm 

 covering the outer surface. Between the two is the supporting 

 layer (mesoglrea), a structureless membrane without cells and hence 

 not a body layer. Both layers consist of epithelial muscular cells 

 (ef. p. 02), the basal ends of which are produced into smooth 

 muscle fibres, those of the ectoderm running lengthwise, those of 

 the entoderm around the body. The ectoderm further contains 

 ganglion, nettle aiul sex cells. The nettle cells on the tentacles 



