16 THE VOYAGE OF H.M.S. CHALLENGER. 
pinnate. In such forms of ramification the primary pinne are almost always opposite, 
while the ultimate pinnz or hydrocladia are alternate. Sometimes, however (Pl. XIX. 
fiz. 4), the primary pinnee are also alternate. 
In some rare cases (Pl. I. fig. 5) the hydrothecee-bearing pinne are themselves pin- 
nately branched, the branches also carrying hydrothece, like the pinne from which they 
spring. 
Definitions of the principal terms used in the descriptive morphology of the Hydroida 
Hyprosoma.—The entire hydroid colony. 
KcropErM.—The most external of the three organised layers of which the body of 
every hydroid is composed. 
EnpoperM.—The most internal of the three organised layers of which the body of 
every hydroid is composed. 
MersoperM.—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. 
Prrisarc.—The unorganised chitinous excretion by which the soft parts are to a 
oreater or less extent invested. 
Zoorps.—The more or less independent products of non-sexual reproduction, forming 
by their association the hydroid colony. 
TropHosoMe.—The entire assemblage of such zooids as are destined for the nutrition 
of the colony. 
GonosomE.—T'he entire assemblage of such zooids as are destined for the sexual 
reproduction of the colony. 
Hyprantus.—The proper nutritive zooids, or those which carry the mouth and proper 
digestive cavity and which are almost always furnished with tentacles. 
HyprotHEca.—The cup-like chitinous receptacles which protect the hydranths in 
the calyptoblastic genera. 
THECOSTOME,—The external orifice of the hydrotheca, which permits of the extrusion 
and retraction of the hydranth. 
InTRATHECAL Ripce.—An imperfect septum by which, in many Plumularide, the cavity 
of the hydrotheca is transversely divided into a proximal and a distal portion. 
Hypropuyton.—The common basis of the hydrosoma by which its zooids are 
connected into a single colony. 
HyproruizA.—The proximal end of the hydrophyton by which the colony fixes itself 
to other bodies. 
HyprocavuLus.—All that portion of the hydrophyton which intervenes between the 
hydrorhiza and the hydrothecee or hydranths. It usually consists of a main stem and 
