DEFINITIONS OF TERMS. 



In the descriptions of the genera and species I have adopted the tenninology 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. 



Undoderm. 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. llie 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 Plumularida; 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 liydranth. It is polysip/ionic or fascicled when it is composed of several mutually adherent tul>es ; 

 jiionosi/thoriic, when consisting of a single tube. In some species the cavity of its perisarc may be divided 

 bj' aimular rid<res or imi)erfect septa, — septal rii/yes. The rachis is that portion of the hy(h-ocaulus along 

 which ill the Piumu'.arida; the hydrotheca; are arranged. 



Coeuosarc. The common organized fleshy portion of the hydrophyton ; the living bond by which 

 the zooids are organically united to one another. 



Nematophores. Peculiar l)odit's 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 Tlumulariilse. They are supracahjcine 

 wlien 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 

 — (i\a 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 hydruids is formed on the summit of the gonangium, 

 where it constitutes a receptacle into which the ova are discharged in order to pass witliin it through 

 some of the earlier stages of their developnient. 



Corbula. A basket-shaped receptacle which encloses groups of gonangia in certain plumularian hy- 

 droids. 



Phylactogouia. Special l)ranches intended for the protection of the gonangia in certain plumularidans. 



Gymnoblastic. The condition of a hydroid when no external ])rotective reoept.aele (hydrotheca or 

 gon:in;;iuMi) invests either nutritive or generaiive buds. Gymnoulastea, the name of one ot' the sub- 

 orders of IIVDRUIDA. 



Calyptoblastic. Tlie condition of a hydroid when an external ])rotective receptacle (hydrotheca or 

 gon;u)'.'iiini) invests either the nutritive or generative buds. Cai.vptohi.astka, the name of one vS 

 the MilHirdcrs of IIvi>l!()ll>A. 



