4 THE VOYAGE OF H.M.S. CHALLENGER. 
Tn accordance with these differences, the great leading groups of the Plumularide may 
be systematically arranged as in the following scheme :— 
Gonangia with special ) ELEuTHEROPLEA 
protective apparatus, PHYLACTOCARPA, 
{ Moveable nematophores 
5 ELEUTHEROPLEA. . : 
| always present, a Gonangia destitute of ) 5, 
5 : : ELEUTHEROPLEA 
A | special protective ap- 
| GYMNOCARPA. 
| {| paratus, 
4 
5 { Gonangia with special | SratopLEa 
= | protective apparatus, PHYLACTOCARPA. 
~ | Moveable nematophores 
4 i ae STATOPLEA. : : 
Ay never present, Gonangia destitute of SanOPTE 
| | special protective ap- Rare 
| paratus, ‘ 
The opportunities of examining the Hydroida in a living state, or even in dead speci- 
mens which have the soft parts sufficiently well preserved, are so comparatively few, that 
the systematist is compelled to have recourse to the chitinous periderm, as affording almost 
the only available grounds for determination and classification. 
This chitinous external skeleton, however, presents so many points of high morpho- 
logical interest, that the classification thus based upon it is by no means an arbitrary and 
artificial one. 
A correct idea of the morphology of the chitinous skeleton in the Plumularide will 
be best obtained by comparing with one another the modifications of form which it 
presents when examined in the various parts of the colony or hydrophyton in the 
different groups. 
Morphology of the Trophasome. 
Hydrocaulus.—The hydrocaulus of the Plumularide, both in the Eleutheropiea and 
the Statoplea, may be formed either of a single tube (monosiphonic) or it may be 
fascicled or polysiphonic. The fascicled condition may exist in the main stem and in the 
principal branches, but the ultimate ramuli or hydrocladia are always monosiphonic. 
The polysiphonic condition gradually gives place to the monosiphonie in passing from 
the proximal to the distal parts of the colony, the fascicled stem sending off bundles of 
tubes to form the branches, and these again throwing off smaller bundles to form 
secondary branches, until finally, in the distal parts of the colony, the fasciculation has 
become exhausted, and a monosiphonic condition has taken its place. 
The tubes which compose the fascicled portions of the hydrocaulus communicate 
laterally with one another. In Aglaophenia coarctata (Pl. XIX. figs. 7-9) this communica- 
tion is plainly seen to be effected by very short tubular processes which are given off from 
