COLLENTERATA 49 
The simplest form of Hydromedusa is represented by Hydra, 
which exists only in the hydroid form. Its reproductive organs 
show no trace of a medusoid nature; and although the medusae 
can be traced in other and more specialised species through 
stages of degeneration till they become little more than pro- 
tuberances on the body-wall full of sexual cells, still there is 
nothing in the ovary and testis of Hydra to warrant the view 
that they are not simply sexual organs. 
In some Hydromedusae a distinct alternation of generations 
is present; that is, the hydroid person produces asexually, by 
budding, a medusoid person which produces sexually, by means 
of ova and spermatozoa, the hydroid person again. Thus asexual 
and sexual modes of reproduction alternate in the life-history 
of these animals, and each mode is associated with a distinct 
kind of animal: the asexual with the hydroid, usually a fixed 
form; the sexual with the medusoid, a free-swimming form. 
This kind of alternation of generation—budding alternating with 
the sexual method—has been termed metagenesis; it occurs in 
many of the lower animals. 
Although the fixed hydroid differs a good deal in appear- 
ance from the free-swimming medusa, they can both be reduced 
to a common type. In both forms of person very considerable 
complexity of form is often combined with great simplicity of 
ultimate structure. Many forms are colonial, and the in- 
dividuals composing the colonies are commonly modified to 
subserve various functions, and thus may become degraded to 
the level of organs. ; 
1. Ectoderm. 12. Flagella. 
2. Endoderm. 13. Vacuole. 
3. Mesogloea. C. Two ectoderm cells. 
4, Layer of muscular processes of 1. Nucleus. 
ectoderm cells cut across just 2. Muscular tails. 
outside mesogloea. D. An endoderm cell of H. viridis. 
5. Interstitial cells. 1. Nucleus. 
6. Cnidoblast containing nemato- 2. Chromatophores. 
cysts. 3. Ingested nematocyst. 
7. Nematocyst. E. A large nematocyst with extended 
8. Cnidocil. barb. 
9. Nucleus of endoderm cell. F. A small nematocyst. 
10. Ingested diatom. G. A spermatozoon. 
11. Pseudopodium. 
A, B, and CG, after Jeffery Parker; D, after Lankester; F and G, after Howes. 
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