CHAPTER III. 



BRANCH III. CCELENTERATA (HYDEOIDS, JELLY- 

 FISHES AND POLYPS). 



General Characters of Coslenterates. In this branch, 

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

 Tubular ia (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 Calenterata 

 (Greek, woz/Vos, hidden, and evrspov, 

 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 

 grades 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 



Fig. 35. AHydrokl, Tubu- 

 laria. m, medusa buds ; ct, 

 tentacles ; p, proboscis. 

 From Tenney's Zoology. 



