We have already seen {see Introduction) that the term "individual," in its zoological sense, must be restricted to "the entire result of the development of a single fertilised ovum," and that in this sense an individual may either be simple, like an Amoeba, or may be composite, like a Sponge, which is produced by an aggregation of amoebiform particles. If all the parts composing an individual remain mutually connected, its development is said to be "continuous;" but if any of these parts become separated as independent beings, the case becomes one of "discontinuous" development. We have seen, also, that however long zooidal multiplication may go on, there ultimately arrives in the history of every individual a period at which sexual reproduction must be called in to insure the perpetuation of the species throughout time. This truth is expressed by Steen-strup's celebrated law of the "alternation of generations."

Amongst the Hydrozoa, the individual may be either simple or compound, and the development may be either continuous or discontinuous, the following terms being employed to denote the phenomena which occur.

Hydrosoma

This is the term which is employed to designate the entire body of a Hydrozoon, whether it be simple, as in the Hydra, or composite, as in a Sertularian.

Polypite

The alimentary region of a Hydrozoon is called a "polypite;" the term "polype" being now restricted to the same region in the Actin-ozoa. In the simple Hydrozoa the entire organism may be called a "polypite ;" but the term is more appropriately applied to the separate nutritive factors which together make up a compound Hydrozoon. By Professor All-man the term "hydranth" is used in preference to "polypite."

Distal And Proximal

These are terms applied to different extremities of the hydrosoma. It is found that one extremity grows more quickly than the other, and to this free-growing end - at which the mouth is usually situated - the term "distal is applied. To the more slowly growing end of the hydrosoma - which is at the same time usually the fixed end - the term "proximal" is applied. These terms may be used either in relation to a single polypite in the compound Hydrozoa, or to the entire hydrosoma, whether simple or compound.

Hydrorhiza

This term is applied to that portion of the proximal end of a Hydroid colony by which it is attached to some foreign body.

Coenosarc

This is the term which is employed to designate the common trunk, which unites the separate polypites of any compound Hydrozoon into a single organic whole.

Polypary

The term "polypary" or "polypidom" is applied to the horny or chitinous outer covering or envelope with which many of the Hydrozoa are furnished. These terms have also not uncommonly been applied to the very similar structures produced by the much more highly organised Sea-mats and their allies (Polyzoa), but it is better to restrict their use entirely to the Hydrozoa. By Professor Allman the term "peri-sarc" is given to the chitinous investment by which the soft parts of the Hydrozoa are often protected.