DEFINITIONS OF TERMS. 



In the descriptions of the genera and species I have adopted the tei-minology 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. Tlie entire assemblage of such zooids as are destined for the sexual reproduction of 

 the colony. 



Hydrantbs. 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 Plumularids the cavity of the hydrotheca 

 is transvcri^ely 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 h\ilrantli. It is poli/nijilionic or fascicled when it is composed of several mutually adherent tubes ; 

 vwnn:<iph<mic, when consisting of a single tube. In some species the cavity of its perisarc may be divided 

 by annular ridges or imperfect septa, — septal riilges. The rachht is that portion of the hydrocaulus along 

 whicli in the I'lumularida; the hydrothecse are an-anged. 



Ccenosarc. The common organized fleshy portion of the hydrophyton ; the living bond b)- which 

 the zooids are organically imited to one another. 



Nematophores. Peculiar Ixidies developed in certain genera from definite points of the hydrosoma, 

 and consisting of a chitinous recejjtacle with sarcode contents in which thiead-cells are usually im- 

 mersed. They are eminently characteristic of the family of the Plumularidae. They are supracalycbie 

 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 iir spcrnuitozoa. 



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 gonangium, 

 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 j)rotection of the gonangia in certain plumularidans. 



Gymnoblastic. The condition of a hydroid when no external protective receptacle (hydrotheca or 

 gonangium) invests either nutritive or generative buds. Gymnoblastea, the name of one of the sub- 

 orders of IIydroida. 



Calyptoblastic. Tlie condition of a hydroid when an external protective receptacle (hydrotheca or 

 gonaniiuni) invests either the nutritive or generative buds. Calyptoblastea, the name of one of 

 the sulioi'dcrs of IIydkoida. 



