THE VOYAGE OF H.M.S. CHALLENGER. 



lu accordance mth these differences, the great leading groups of the Plumularidse may- 

 be systematically arranged as in the following scheme : — 





Moveable nematophores Uleutheroplea. 

 always present, J 



Moveable nematophores ) Statoplea. 

 never present, J 



' Gonangia with special ) Eleutheroplea 

 protective apparatus, / phylactocarpa. 



Gonaneia destitute of ] ^^ 



.°i , ,. I Eleutheroplea 



special protective ap- V 



paratus, j 



gymnocarpa. 



Gonangia with special ) Statoplea 

 protective apparatus, J phylactocarpa. 



Gonangia destitute of I g^^^^p^^^ 



special protective ap- ^ ^^^jj^ocarpa. 

 paratus, } 



The opportunities of examining the Hj^Iroida 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 Plumuhiridae will 

 be best obtained by comparing with one another the modifications of form which it 

 presents when examined in the A^'arious parts of the colony or hydrophyton in the 

 different groups. 



Morphology of the Tropliosome. 



Hydrocauhis. — The hydrocaulus of the Plumularida3, both in the Eleutheroplea 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 monosiphonic 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 ha effected by very short tubular processes which are given off from 



