220 HYDKOID^. Part IV. 



retract toward their bases, even to such an extent as to be only three times longer 

 than thick {Fig. 9, «), whilst the inner surface of the chyraiferous cavity {Fig. 2, F, g) 

 becomes deeply plicated in obliquely transverse folds, which look like spiral semi- 

 partitions. The depth of these folds varies considerably, under the same degree 

 of contraction in the stem ; sometimes they are slight and very oblique (H, g), 

 or present a form intermediate (D, g) between this and the firstrmentioned state. 

 In extremely contracted states {Fig. 9) every thing is so compressed, crowded, and 

 reduced, that the cells of the different layers are not distinguishable as cells, but 

 appear like coarse granules more or less regularly disposed. 



The creeping stem {Fig. 2, e) is covered by a dense, rigid, horny sheath, which 

 rises a short distance, on the narrow bases of the upright stems, in the fonn of 

 shallow cups, and then suddenly thins out into a mere film which vanishes at 

 a little distance above. This leaves the whole upright stem, even to its slender 

 base, full freedom to assume any shajie it may choose. The whole extent of the 

 colony is composed of two distinct, continuous layers. The outer layer or wall 

 {Figs. 2, D F G H /, and 9, /) of the upright stem and head, is moderately thick, 

 and very transparent. At the mouth {Fig. 9, g) it terminates abruptly, in the 

 simple lip. On the tentacles {Fig. 8, a) it has the same proportionate thickness, 

 without increasing, in this respect, at the blunt tips of these organs, and is still 

 more transparent than on the stem and head. The inner wall (/\) of the head 

 and stem is about twice as thick, or a little more, than the outer wall, and much 

 denser in appearance. This wall also terminates abruptly at the mouth {Fig. 9, g), 

 on the same level with the outer wall. The interior surface of this wall is covered 

 by brownish-red granules, which constantly become detached, and are carried away 

 by the flow of the chymiferous fluid, which circulates backward and forward in 

 the upright stems and the stolons. The remarkable oblique folds which appear 

 in this wall, when the stem is contracted, have already been mentioned above ; 

 we have, however, to point out here, in addition, a few broad, longitudinal folds 

 in the head, just below the mouth {Fig. 9, g). These vanish, however, at a short 

 distance below. In the tentacles {Fig. 8, a) the inner wall of the body is con- 

 tinuous, as a solid axis, which is composed of a single row of very large trans- 

 parent cells. 



From the colony partially represented in PI. XXI. Fig. 2, it is evident that 

 the individuals united together by the creeping stem from Avhich they arise, are 

 very unequal in their development, some (E) having very few tentacles and no 

 sign of raedusaj-buds, while others (A) have their full number of tentacles and bear 

 large bunches of medusfe. In their activity they show also a marked independence, 

 some being fully expanded, while others are more or less contracted. 



