CCELENTERATA. yf 
the Actinozoa. In the Hydrozoa, there is no body-cavity dis- 
tinct from the digestive cavity—or, in other words, the body- 
cavity zs the digestive cavity. In the Acéznozoa, on the other 
hand, there is a distinct digestive cavity, but this opens directly 
into the general body-cavity, so that the two form distinct but 
freely communicating divisions of the same chamber. 
CrLass I. HYDROZOA. 
The Aydrozoa are defined as Celenterata in which the walls 
of the digestive sac are not separated from those of the general 
cavity of the body, the two coinciding with one another. The 
reproductive organs are external, in the form of outward pro- 
cesses of the body-wall (fig. 16, a, 6). 
The Aydrozoa are all aquatic in their habits, and, with the 
exception of two genera, all are inhabitants of salt water. The 
class includes both simple and composite organisms, of which 
the most familiar are the sea-firs and their allies (Hydroid 
zoophytes), the fresh-water polype or Hydra, the sea-jellies 
(Meduse), and the Portuguese man-of-war (Physalza). Owing 
to the extremely complicated nature of many of the ydrozoa, it 
appears advisable to preface their description by an explanation 
of some of the more important terms which are employed in 
connection with various members of the class. 
GENERAL TERMINOLOGY OF THE HYDROZOA. 
Individual—tIn order to understand fully the meaning which 
is attached to the term “individual” in zoological language, 
it is necessary to glance briefly at the general features of 
reproduction as displayed in different sections of the animal 
kingdom. Reproduction is the process by means of which new 
individuals are produced and the perpetuation of the species 
insured, This end may be attained in various ways, but these 
all come under the two heads of “sexual” and “non-sexual” 
reproduction. In sexwal reproduction, by which alone can 
fresh beings be produced amongst the higher animals, the 
essential element of the process consists in the formation of 
two distinct structures, a germ-cell or ovum, and a sperm-cell 
or spermatozoid. By the union of these distinct reproductive 
elements, fresh beings can be produced. As a general rule, 
the germ-cell is produced by one individual (female), and the 
sperm-cell by another (male); but amongst the lower animals 
