CHAPTER III. 



BEANCH III.- 



-CCELENTEEATA (Hydeoids, 

 Fishes and Polyps). 



Jelly- 



■i:/"'. 



General Characters of Coelenterates.— In this branch, 

 which is represented by animals hke the Hydra (Fig. 36) and 

 Tuhularia (Fig. 35), the body consists 

 of three cell-layers, surrounding a 

 definite, single, digestive cavity, the 

 mouth of the cavity being surrounded 

 by a circle of tentacles, which are hol- 

 low and connect directly with the 

 stomach. The latter, however, is only 

 partly differentiated or set apart from 

 the body, hence the name Ccelenterata 

 (Greek, uoikoi, hidden, and evrepov^ 

 digestive tract). From the stomach 

 often radiate water-vascular canals, no 

 blood-system yet appearing thus far in 

 the animal kingdom, the products of 

 digestion reaching the tissues from 

 the smaller branches of the primary 

 water-vascular canals. The nervous 

 system is either absent, or in different 

 gTades of development, from the iso- 

 lated nervo-muscular cells of Hydra 

 and the scattered nerve-cells of an 

 Actinia, to the continuous ganglion- 

 ated nervous ring of the minute 

 jelly-fish such as Sarsia. These animals display a striking 

 amount of radial symmetry, the organs and body being dis- 

 posed in a radiate manner around a central vertical axis, in 



Pig. 35.— AHydroid. Tu6u- 

 laria. m, medusa budw ; cf, 

 tentacles ; p, proboscis.— 

 From Tenney's Zoology. 



