16 THE VOYAGE OF H.M.S. CHALLENGER. 



pinnate. In such forms of ramification the primary pinnae are almost always opposite, 

 while the ultimate pinnae or hydrocladia are alternate. Sometimes, however (PI. XIX. 

 fig. 4), the primary pinnse arc also alternate. 



In some rare cases (PL L fig. 5) the hydrothecse-bearing pinnae axe themselves pin- 

 nately branched, the branches also carrying hydrothecae, like the pinnae from which they 

 spring. 



Definitions of the principal terms used in the descriptive morphology of the Hydroida 



Hydrosoma. — The entii'e hydroid colon}-. 



Ectoderm. — The most external of the three organised layers of which the body of 

 every hydroid is composed. 



Endoderm. — The most internal of the three organised layers of whicli the body of 

 every hydi'oid is composed. 



Mesoderm. — An organised layer which lies between the ectoderm and endoderm. 

 The above are the three embryonic layers which are persistent, and admit of being 

 recognised during the life of the hydroid. 



Perisarc. — The unorganised chitinous excretion by which the soft parts are to a 

 greater or less extent invested. 



ZooiDS. — The more or less independent products of non-sexual reproduction, forming 

 Ijy their association 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 furnished with tentacles. 



Hydrothece. — The cup-like chitinous receptacles which protect the li3-drantlis in 

 the calyptoblastic genera. 



Thecostome. — The external orifice of the hydi-otheca, which peimits of the extrusion 

 and retraction of the hydi-anth. 



Intrathecal Eidge. — An imperfect septum by which, in many Plumularidce,. the cavity 

 of the hydrotheca is transversely divided into a proximal and a distal portion. 



Hydrophyton. — The common Ijasis 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 hycbophyton which intervenes between the 

 hydrorhiza and the hydi-otheca^ or hydranths. It usually consists of a main stem and 



