208 CCELEXTERATA 



In comparison with the sponges the Ccelenterata may be called epi- 

 thelial organisms. A mesoderm (mesogla-a) may be entirely lacking or 

 may have but a subordinate development. The ectoderm and entoderm, 

 on the other hand, are the important tissues producing 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 frequently occur in the life history, one agreeing well in struc- 

 ture 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 Jiydropolyp, forming an important 

 archetype from which all other conditions medusas, scyphopolyp, and 

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

 Hydra. The body (fig. 169) is a sac, the closed end of which, the pedal 

 disc, is used for attachment. The other end bears the mouth which leads 

 to the gastrovascular (digestive) cavity. Around the mouth is a circle of 

 tentacles used in capturing food. 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. 170), an entoderm of flagellate cells 

 lining the gastrovascular space, and the ectoderm covering the outer sur- 

 face. Between the two is the supporting layer (mesoglcea), a membrane 

 without cells and hence not a body layer. Both layers consist of epithelial 

 muscular cells (cf. p. Si), 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 are crowded into 

 small ridges or ' batteries.' The sex cells (at certain times) produce swell- 

 ings on the column; a circle of male swellings close beneath the tentacles, 

 the female cells farther down the column (fig. 169). Individuals reprodu- 

 cing by budding are more common than the sexually mature (fig. 93). 

 Small elevations appear on the column, enlarge, form tentacles, and at 

 last a mouth, after which they may separate from the parent. 



