i ZOOLOGICAL EXERCISES. 
motionless, and are not provided with a long filiform body, 
When both male and female organs are found in the same 
individual the species is said to be hermaphrodite ; when 
they are found in different individuals the species is said to 
be diwcious. The union of one or more spermatozoa with an 
ovum is called impregnation, and the ovum is then said to 
be fertilised. In ordinary sexual reproduction this union is 
necessary for the production of a new individual, but in 
some cases ova are known to develop without the contact of 
spermatozoa. This is called parthenogenesis. Conjugation 
is the coalescence of two similar cells to produce a new 
individual like themselves. IJmpregnation is the coalescence 
of two dissimilar cells to produce a new individual unlike 
either of them. Conjugation is common among the lower 
plants, but in animals it is only found in the Protozoa. 
The muscular and nervous systems, by which animals 
are brought into relation with the outer world, are very 
closely related, and in the hydra are actually different 
portions of the same cell. The chief use of the muscular 
system is to procure food and distribute it over the body ; 
while it is the stimulus supplied by the nerves that causes 
the muscles to contract. 
Two kinds of Infe-—In every animal higher than the 
Protozoa two kinds of life co-exist: (1) the life of each cell or 
tissue life, and: (2) general life. This latter, which is 
ordinarily called life, is due to a combination or co-ordination 
of the individual cells, and if this co-ordination is broken by 
the destruction of any essential organ, general life ceases ; 
tissue life, however, still goes on, until the cessation of the 
supply of nourishment gradually kills each cell. On the 
other hand, while the general life is in full vigour, each cell 
has an independent life of its own—each is born, lives, and 
dies without affecting the general life. It is a life within a life. 
