230 C(ELENTERATA. 



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

 Sexual and asexual types of reproduction can be combined in the 

 same species, producing an alternation of generations. 



In comparison with the sponges the Co3lenterata may be called 

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

 lacking or may have but a subordinate development. The ectoderm 

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

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

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



Class I. Hydrozoa (Hydromedusae). 



According to varying standpoints the Hydrozoa can be placed 

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

 former group two forms are frequently introduced into the life 

 Mstory, 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 free-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 medusae; the medusa, on the other hand, is 

 the sexual stage, and from its eggs polyps arise. 



The polyp of the Hydrozoa is the hydropolyp, forming in the 

 branch of coelenterates an important archetype from which all 

 other conditions medusae, scyphopolyp, and even the coral polyp 

 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 (mesogloea), a structureless membrane without cells and hence 

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

 (cf. p. 92), 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 and sex cells. The nettle cells on the tentacles 



