CHAPTER IV 
COELENTERATA 
CHARACTERISTICS.— Acoclomata 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. HYDROMEDUSAE 
. SCYPHOMEDUSAE 
B 
C. ACTINOZOA. 
D. CTENOPHORA. 
; = Hydrozoa. 
Crass A. HYDROMEDUSAE. 
CHARACTERISTICS.— The Hydroid form may be free or sessile, single 
or colonial. It is rarely without tentacles, which are nearly 
always solid. A horny perisare or a calcareous skeleton may 
be developed. Asexual reproduction by gemmation usually 
takes 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. No 
gastral filaments are present. The medusa may arise directly 
JSrom the egg, but this is rare. 
