HYRACTINIA ECIIINATA. 221 



substance abundance of thread-cells. The whole free surface of the connuon basal expansion of 

 the colony thus presents an absolutely naked layer of ectoderm. 



Over the entire surface short blunt conical spines spring at short intervals (PL XV, fig. 1). 

 These spines consist of a chitinous framework, continued from the general chitinous system of the 

 common basis, and overlaid and penetrated by an extension of the ccenosarc. 



The framework of each completely formed spine consists of a conical process of chitine {fig. 7), 

 whose surface is traversed, from the base to the apex, by longitudinal jagged ridges, which at the 

 base of the spine are continuous with the ridges of chitine, between which, as already said, the 

 superficial coenosarcal canals are lodged. A transverse section close to the base of a W'Cll-deve- 

 loped spine shows it to contain numerous cavities, which are here arranged in more or less regularly 

 concentric series (fig. 4). These cavities diminish in number and regularity towards the middle 

 of the spine, while close to the summit they have entirely disappeared, and the longitudinal ridges 

 here meet in the axis (fig. 5). 



The cavities of the spine intercommunicate with one another, like those of the common 

 horizontal layer, and, like them, each is filled with a mass of cocnosarc, having a central cavity 

 surrounded by cells filled with coloured granules. The furrows occupying the intervals of the 

 longitudinal ridges are also filled with ccenosarc (figs. 4, h, .5, and 7, h), which is here excavated 

 into similar cavities, while they are closed in by a layer of distinct thread-cell-bearing naked 

 ectoderm, which thus invests the whole spine, precisely as in the common horizontal basis. 



It is plain from this that the structm-e of the spine is in all essential points the same as that 

 of the horizontal basis, chiefly diS"ering from it in the greater development of its chitinous element, 

 which acquires much greater thickness and shows very evident layers of successive deposition. 

 Towards the mouth of the shell the spines are but slightly developed, and are here little more 

 than short, blunt, conical tubercles (fig. 1). 



There can be no doubt that the whole hydrophyton of Ili/dracf'mia must be regarded as 

 consisting of a set of ccenosarcal, freely intercommunicating tubes, which have excreted from their 

 sm-face a chitinous perisarc, and have intimately coalesced with one another. 



The reproduction of the hydrophyton may be watched in a mutilated specimen, kept alive in 

 a jar of sea- water; and it will be then seen to be at first a simple network of ccenosarcal 

 tubes (PI. XVI, fig. 11) invested by a chitinous perisarc, and, as Dr. Strethill Wright first 

 pointed out, presenting very distinct currents in the contained somatic fluid. It is obvious that 

 there is in this stage no essential difference between the hydrophyton of Hi/dractinia and that of 

 other hydroids with undeveloped hydrocaulus. The meshes of this network, ho\\'cver, are 

 ultimately obliterated by the thickening and coalescence of their chitinous walls. 



The Ili/dranths and Spiral Zooids. — Prom the whole of the naked surface of the ccenosarc, 

 whether covering the horizontal layer or extending over the spines, the hydranths arise ; while 

 the spiral zooids, to be presently described, are confined to a narrow space along the extreme edge 

 of the colony. The somatic cavity of both hydranths and spiral zooids is in direct communication 

 with the system of canals which traverse the ccenosarc in every direction, while their ectoderm is 

 directly continuous with the naked superficial ectoderm of the ccenosarc, and their endoderm 

 with the endodermal portion of this same common basis of the colony. 



The hydranths (PI. XV, fig. 1, ct, a, a) are eminently contractile, and distinct longitudinal 

 muscular fibres may be traced in them through their entire length. When fully extended thcj- 

 have the form of long, attenuated, nearly cylindrical columns, dilated towards the summit; 



