20S CCELEXTERATA 



In comparison with the sponges the Cwlenterata ma)' be called epi- 

 thelial organisms. A mesoderm [mcsogla-a) may be entire!)' lacking or 

 mav 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 cnida\ Hence the group is often called 

 Diploblastica — twodayered animals. 



Class I. Hydrozoa (Hydromedusae) . 



According to varying standpoints the Hyilroz.oa can be placed either 

 higher or lower than the Antliozoa in the system, since in the former group 

 two forms frequentl) occur in the life history, one agreeing well in struc- 

 ture with the Anthozoa, the other standing on a higher grade. The tirst is 

 the sessile and usually colonial polyp, the second the free-swimming medusa, 

 well provided with sense organs. These are usually related to each otlier 

 by an alternation of generations. The polvp is asexual and by budding 

 produces medusa:; the medusa, on tlie otlier hand, is the sexual stage, and 

 from its eggs polyps arise. 



The pol\p of the Hydrozoa is the liydropolyp, forming an important 

 archetype from which all other conditions — medus;u, scyphopolyp, and 

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

 Hydra. The body (lig. idg) is a sac, the closed entl of wliich, 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 tlie circle and a column 

 constituting the rest of the outer wall. 



Hydra has but two boil)' layers (tig. 170), an entoderm of llagellate cells 

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

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

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

 muscular cells (cf. p. 81), the basal ends of which are produced into 

 smooth muscle filires, those of the ectoderm running lengthwise, those of 

 the cntodernr around the body. The ectoderm further contains ganglion, 

 nettle and sex cells. The nettle cells on the tentacles are crowded into 

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

 ings on the column; a circle of male swellings close l)eneath the tentacles, 

 the female cells farther down the column (tig, lOg), Indiviiluals reprodu- 

 cing by budding are more common than the sexually mature (lig, c)^). 

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

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



