Chap. VII. CLYTIA POTERIUM. 299 



immovable substance. Oftentimes it is found attached to the antennoe of the 

 spider-crab (Libinia canaliculata). The main stem {Fig. 2, d) never rises to an 

 erect position, but always creeps, stolon-like, over the body to which it is attached, 

 while the pedicels («) of the sterile hydrae stand up like the stems of the Tubu- 

 larians. These pedicels are always simple, varying from three, to four sixteenths of 

 an inch in height, and are faintly ringed from top to bottom. The rings are not 

 so strongly marked as in some other species, but are, more properly, slight waves 

 {Fig. 3, <^). At the top of the pedicel there is, however, one well-developed ring 

 {c% upon which the calycle rests. The calycle (c^) is deeply campanulate, has 

 a smooth edge, and its depth is to the breadth as four to three. The wall of 

 the cup is very thick, in fact three or four times as thick as the wall of the 

 pedicel; it thins out suddenly at the edge, but at the lower part it abruptly 

 doubles its thickness, to form a diaphragm (c). This diaphragm, or transverse 

 semi-partition, is altogether different from that of any of the upright and branching 

 forms of the Campanularians ; in profile it has the form of an equilateral triangle, 

 of which one side is based upon the calycle, and the other two sides are free, one 

 facing toward the mouth of the cup, and the other, obliquely, toward its base. 

 The free edge is rounded, and the inferior face is concave, so that it embraces 

 a spherical cavity. 



The reproductive hydra? {Fig. 2, h) do not seem to have any systematic arrange- 

 ment upon the creeping stem, but arise indiscriminately with the pedicels of the 

 hydrae. Their calycles have an oval cylindrical shape, and are about twice the 

 length of the calycles of the hydrae. They are more or less wavy in outline, 

 and frequently have the appearance of being ringed (PI. XXIX. Fig. 5). Their 

 aperture (PL XXVIII. Fig. 16, /c^) is truncate, and smooth, and occupies nearly the 

 whole breadth of the calycle. The base of the calycle tapers into a short pedicel 

 {Figs. 15, 16, and 19, a), which rises immediately from the creeping main stem. 

 The wall of this calycle is of a uniform thickness throughout {Fig. 15, k ; PI. XXIX. 

 Fig. 2), and, in this respect, agrees with the pedicel upon which it is based. 



The sterile hydra? (PI. XXVIII. Figs. 2, a, and 3, a) have twenty-four tentacles, 

 exhibiting the same proportions and structural elements as most Campanularians. 

 Compare PI. XXXIII. Fig. 5, and PI. XXX. Figs. 4 and 5. It must be borne in 

 mind, however, that the hydra of this species bears somewhat different relations 



Sertidaria, Gmelin, Linn. Syst, Nat., 1788, p. 3844. Sertularia, Oken, Lehrbuch der Naturgeschichte, 



" Abildgaard, in Zool. Danica, 1789, III. 1815, III. p. 92. 



p. 61. " Bosc, Hist. Nat. des Vers, 1830, III. 



" Turton, British Fauna, 1807, p. 212. 2de ed., p. 94. 

 « Olivi, Zool. Adriatica, 1792, p. 288. 



