Chap- ^II- OBELIA COMMISSURALIS. 31^ 



of the calycle is the same as that of the hydra. The wall (k) of this calycle 

 IS of uniform thickness throughout, and, in this respect, is equal to that of the 

 peduncle. The microscopic structure of the horny sheath of the hydrarium does 

 not differ from that of other Hydroids; it being merely a concentric series of 

 fibrillated lamellae (PI. XXXIII. F^r;s. 13 and 14, c^). The transverse stria,, which 

 appear here and there in the thickness of the sheath, are inexplicable features, 

 which have all the appearance of minute anastomosing vessels in another genus 

 (PI. XXXiy. Fiff. 1, a' a^ There is no essential difference in the hydra from 

 that of Laomedea amphora (PI. XXX. Fi.s. 4 and 5), except that, perhaps, the 

 latter has more tentacles, but of this we are not certain. Every calycle contains 

 a single hydra, consisting of a digestive cavity (PI. XXXIII. F/>/. 5, y), which is based 

 upon the semi-partition mentioned above (PI. XXXllI. Fiff. 12, c), and terminated 

 by a single coronet of slender, tapering tentacles (PI. XXX. Fiff. 5, A B) with 

 an extremely dilatable simple proboscis (pr) rising in the centre of the circlet 

 As peculiar to all Campanularians, the tentacles are here, also, alternately elevated 

 (B) and depressed (A) when fully expanded, although their bases are, strictly in one 

 row. The double walls of the stem (PI. XXXIII. Fi^. 5, «> //), the digestive cavity 

 {a b\ the tentacles («^ b% the proboscis («« h% the reproductive calycle (PL XXXIY 

 F,g. 11, (i y), and even the medusa3-buds {(r /), are continuous with each other 

 throughout the hydromedusarium. The inner wall is generally twice as thick as 

 the outer one, except, perhaps, in the reproductive calycles, where the two (PI 

 XXXIV. Fiff. 11, li y) are nearly alike. The flexibility of the semi-partition 

 (PI. XXXIII. Fffs. 12 and 12^ .) allows the passage of the double walls (% 5 

 a' P) of the pedicel into those of the digestive cavity (a h), without sensible con- 

 striction. The lasso-cells are arranged not only in transverse but also in lono-itudinal 

 rows (PI. XXXII. Fig. 5^ I), the transverse rows corresponding to the tmnsverse 

 walls of the axial cells {b% There are at least six longitudinal rows of these pre- 

 hensile organs, and there are no other Hydroids in which the individual lasso-cells 

 project so flir from the surface as in this species, and in all the Campanularians 

 The figure which we give here {Fig. 5^) represents two of the lasso-threads wound 

 about an Infusorium («), which was caught while we were examining the tentacle 

 As we have said before, the walls of the reproductive calycles are double 

 and m direct prolongation of those of the stem. They are supported by processes 

 (PI. XXXIV. Fig. 11, D from the outer wall, and by their terminal attachment 

 ir, Y) to the end of the calycle (P F). The outer surfixce of the exterior wall 

 is exceedingly tran.sparent, and, being elevated in longitudinal ridges {(f), has the 

 appearance, in profile, of a third wall ; and, moreover, being very plastic, i't doubles 

 over the medusa? and seems to form an exterior sheath or veil. The medusa? 

 (A-G) occupy the whole length of the axis, and are present in the younger sta-es 



