DEFINITIONS OF TERMS. 
In the descriptions of the genera and species I have adopted the terminology which I have used 
on other occasions, and these descriptions will perhaps be rendered more intelligible by giving here 
definitions of the principal terms employed. 
Hydrosoma. The entire hydroid colony. 
Ectoderm. The more external of the two organized layers of which the body of every hydroid 
is composed. 
Endoderm. The more internal of the two organized layers of which the body of every hydroid 
is composed. 
Perisarc. The unorganized chitinous excretion by which the soft parts are to a greater or less ex- 
tent invested. 
Zooids. The more or less independent products of non-sexual reproduction, forming by their asso- 
ciation the hydroid colony. 
Trophosome. ‘The entire assemblage of such zooids as are destined for the nutrition of the colony. 
Gonosome. The entire assemblage of such zooids as are destined for the sexual reproduction of 
the colony. 
Hydranths. The proper nutritive zooids, or those which carry the mouth and proper digestive 
cavity, and which are almost always set with tentacles. 
Hydrotheca. The cuplike chitinous receptacle which protects the hydranth in the Calyptoblastic 
genera. 
Intrathecal Ridge. An imperfect septum by which in many Plumularide the cavity of the hydrotheca 
is transversely divided into a distal and a proximal portion. 
Hydrophyton. The common basis of the hydrosoma by which its zooids are connected into a 
single colony. 
Hydrorhiza. The proximal end of the hydrophyton by which the colony fixes itself to other 
bodies. 
Hydrocaulus. All that portion of the hydrophyton which intervenes between the hydrorhiza and 
the hydranth. It is polysiphonic or fascicled when it is composed of several mutually adherent tubes ; 
monosiphonic, when consisting of a single tube. In some species the cavity of its perisare may be divided 
by annular ridges or imperfect septa,— seplal ridges. The rachis is that portion of the hydrocaulus along 
which in the Plumularide the hydrothece are arranged. 
Ceenosare. The common organized fleshy portion of the hydrophyton; the living bond by which 
the zooids are organically united to one another. 
Nematophores. Peculiar bodies developed in certain genera from definite points of the hydrosoma, 
and consisting of a chitinous receptacle with sarcode contents in which thread-cells are usually im- 
mersed. They are eminently characteristic of the family of the Plumularide. They are supracalycine 
when situated one on each side of the orifice of the hydrotheca; mesial when situated on the mesial line 
of the hydrotheca or rachis. 
Gonophore. The ultimate generative zooid which gives origin directly to the generative elements, 
— ova or spermatozoa. 
Gonangium. An external chitinous receptacle within which in the Calyptoblastic genera the gono- 
phores are developed. 
Acrocyst. An external sac which in certain hydroids is formed on the summit of the gonancium, 
where it constitutes a receptacle into which the ova are discharged in order to pass within it through 
some of the earlier stages of their development. 
Corbula. A basket-shaped receptacle which encloses groups of gonangia in certain plumularian hy- 
droids. 
Phylactogonia. Special branches intended for the protection of the gonangia in certain plumularidans. 
Gymnoblastic. The condition of a bydroid when no external protective receptacle (hydrotheca or 
gonangium) invests either nutritive or generative buds. GyYMNOBLASTEA, the name of one of the sub- 
orders of Hyproipa. 
Calyptoblastic. The condition of a hydroid when an external protective receptacle (hydrotheca or 
gonangium) invests either the nutritive or generative buds. CAaLypTroBLASTEA, the name of one of 
the suborders of Hyprompa. 
