224 ANATOMY OF SPECIAL FORMS. 



The condition of the perisarc is peculiar in that part of the hydroculaus which lies 

 immediately below the hydranth (fig. 1). It here I)ecomes suddenly thin and transparent, and is 

 separated by a wide interval from the contained coenosarc. The ccenosarc, however, is kept in 

 connexion with it by regularly disposed, radiating offsets of its ectoderm. It would seem to be 

 this condition which both Alder and Wright have described as an inner annulated jjcrisarcal 

 tube, surrounded at an interval by an external smooth and thinner one. In my specimens no 

 internal tube of perisarc was apparent. 



The Hydrantlis. — The hydranths are proportionately large, and when extended are nearly 

 cylindrical. They are very contractile, and when in extreme contraction assume the form of a 

 thick club. The tentacles are very numerous and scattered over the body ; they present the 

 usual septate condition of their endoderm, and terminate each in a spherical capitulum loaded 

 with thread-cells. Those hydranths which carry gonophores appear to be in every instance 

 somewhat less developed than the others, though never reduced to the condition of blastostyles. 



The endoderm of the hydranth body (fig. 5) presents a peripheral zone of large prismatic 

 cells (c), and containing a clear colourless protoplasm, having their longer diameter transverse to 

 the axis of the hydranth, and an axial zone {d) of spherical and much smaller cells containing 

 pale reddish granules. Between the endoderm and ectoderm lies a very distinctly fibvillated 

 tissue {b). In the ectoderm («) no distinct cellular structure can be demonstrated, though 

 obscure indications of cells may become apparent under a high power and carefully adjusted 

 compression. Numerous thread-cells may be seen imlDcdded in it. Besides thread-cells of the 

 ordinary form, there occurs here a large oval form of thread-cell in which the coils of the 

 contained filament are very regular and distinct (figs. 9, 10). After evolution the cxscrted 

 portion is seen to present some curious and suggestive modifications of the form more common 

 in the Hydroida. These have been fully described in a former part of the present work 

 (see p. 119). 



The Planoilasfs. — It is in the planoblasfs (fig. 3) that the most striking characters are to 

 be found. They spring from the body of the hydranth in a group near the proximal limit of the 

 tentacles. They arc phanerocodonic, and among the whole of the hydroid medusae I know of 

 none more remarkable than those of Gemmaria. 



The umbrella is deep bell-shaped, with four radiating canals and two opposite marginal 

 tentacles, with large bulbous bases, destitute of ocellus, while the place of two other tentacles is 

 taken by a small bulbous dilation of the distal extremity of each of the intervening radiating 

 canals. The axis of the tentacle is traversed by an uninterrupted tube from its base to its 

 summit (fig. 4). Lying immediately over each of the four radiating canals, and running in 

 the walls of the umbrella close to the outer surface, is an elongated fusiform sac. It seems to 

 originate by its narrow end in the circular canal, and then running exactly parallel to the 

 radiating canal extends over about one fourth part of a meridian of the umbrella. It is filled 

 with thread-cells immersed in a clear flnid (fig. G). These singular sacs admit of a comparison 

 wdth the superficial ridges loaded with thread-cells which Gegenbaur describes as running from 

 the summit of the umbrella to the codonostome in his Zanclea cost at a ; while similar l)ut much 

 smaller sacs, also filled with thread-cells, are developed in the umbrella-walls of the little medusas 

 which constitute the genus Willia of Forbes. 



Imbedded in the substance of the umbrella just within its margin are some oval thread-cells 

 with the contained filament in distinct spiral coils. They are disposed in a line parallel to the 



