CHAPTEE IV 



COELENTEEATA 



Chaeacteeistics. — Acoelomata with a definite shape. The body 

 is usually radially symmetrical about an axis which passes 

 through the mouth. The ectoderm is separated from the en- 

 doderm by a middle layer — the mesogloea — which may be 

 structureless and devoid of cells, or may contain numerous 

 cells. The tissues are not pierced by a series of pores, and 

 there are no collar cells. Protective organs known as nematocysts 

 are characteristic of the group, with the exception of almost 

 all the Ctenophora. Alternation of generations is common, 

 and also the formation of colonies by budding. All the 

 members of the group are aquatic, and most of them marine. 



This group is divided into four classes : 



A. Hydeomedusae ) ,, j 



T. o r =Hydrozoa. 



B. Scyphomedusae j "' 



C. ACTINOZOA. 



D. Ctenophoka. 



Class A. HYDEOMEDUSAE. 

 Chaeacteeistics. — The Hydroidform may be free or sessile, single 

 or colonial. It is rarely without tentacles, which are nearly 

 always solid. A horny perisarc or a calcareous skeleton may 

 be developed. Asexual reproduction by gemmation usually 

 taJees place, the hydroid form budding off a medusiform sexual 

 individual. The 'Medusa has a velum-, and a double nerve 

 ring. The sensory organs are ocelli and otocysts or modified 

 tentacles. The Hydromedusae are bisexual, and the sexual 

 cells are typically ectodermic, sometimes endodermic. iVb 

 gastral filaments are present. The medusa may arise directly 

 from the egg, but this is rare. 



