26 MORPHOLOGY. 



it may be partially or totally witlulrawn, and from whicli it may again spontaneously extend itself 

 {Campamdaria, Sertdaria, &c.). To this receptacle, whicli is characteristic of a large section of 

 the llydroida, the name of hjdrotheca has been given by Huxley. 



The hydrotheca is formed of a chitinous membrane continuous below with the perisarc or 

 common chitinous investment of the hydrophyton, to be presently described. Its margin may be 

 quite even, or it may be divided into minute teeth (woodcut, fig. 2) ; the cup may be permanently 

 open (woodcut, fig. 2), or it may be provided with a kind of operculum formed of triangular 

 segments, united, as by a hinge, to the margin, and capable of completely closing over the cup 

 when thehydranth is retracted (woodcut, fig. 3). Sometimes the operculum consists of a flexible 

 membranous continuation of the cup-margin, and then, during the retracted state of the hydranth, 

 falls together in loose folds. 



In a few hydroids {Bimeria, PL XII, fig. 1) the chitinous perisarc is continued from the 

 hydrophyton for a greater or less distance over the hydranth, which it closely invests without 

 forming anything like a free hydrotheca. In others (various species of BougainviUia, PI. IX 

 and X) the perisarc continued over the hydranth invests it so loosely that in extreme con- 

 traction the hydranth seems to be withdrawn into a cup ; but this apparent cup has nothing of 

 the permanent form or rigid texture of a true hydrotheca, and is always thrown into more or 

 less distinctly marked transverse folds or rugse by the contraction of the hydranth. 



3. Ilijdroplnjion. 



The term lijjdrophjton is used to designate the common basis by which the various zooids of 

 the hydrosoma, or general colony, are kept in union with one another. 



Cmiosarc and Perisarc. — The hydrophyton consists mainly of a fleshy tubular basis, com- 

 posed, like all the zooids which it supports, of an ectoderm and an endoderm. I shall designate 

 this fleshy and only essential portion of the hydrophyton by the name of ccenosarc. 



In every memljer of the Hydroida, however, with whose trophosome we are acquainted, 

 excepting only the fresh-water Hydra, and possibly also Nemopsis and Acaidis, whose trophosomes, 

 like that of Eijdra, are stated — though without sufficient evidence — to be free, the ectoderm 

 excretes from its outer surface an unorganised pellicle, chemically identical with chitine, and 

 forming an external tubular investment for the soft organised ectoderm." This unorganised 

 excretion, which must be placed in a totally different category from that of the ectoderm and 

 endoderm, I shall designate as the perisarc. In some cases it is confined to the hydrophyton ; in 

 others it extends, not only over the entire hydrophyton, but is continued for a greater or less extent, 

 and in a more or less modified form, over the various zooids of the colony. In the Sertularinoi 

 and Campanularinm it forms the cup-like receptacles or liydrothecce already descrilied, into which 

 the hydranths are retractile; as well as pecuhar receptacles — \ki& (jonangia (woodcuts, fig. 3,/, and 

 tig. 3^ fj) — destined for the protection of the sexual buds. It varies greatly in thickness, from a 

 tough investment, in which numerous layers of deposition can be detected, to a delicate, scarcely 

 recognisable pellicle, and is invariably absent from those zooids (fig. 3, g) which have detached 

 themselves from the colony in order to lead an independent life in the open sea. In the adult 

 Hydractinia and Podocoryne it presents the very exceptional condition of not only investing the 



' If future observatious should confirm the view entertained by Agassiz, as to the hydroid nature 

 of the tabulate and rugose corals, wc shall then have examples of calcareous as well as chitinous 

 skeletons amone the Hydroida. 



