Chap. VII. DYNAMENA PUMILA. 329 



is disposed in a uniform series. Immediately beyond the bases of the calycles, 

 the main stem suddenly contracts to its original thickness, and proceeds to the 

 base of the next pair of calycles, where it diverges in two opposite directions as 

 before, and so it traverses the whole length of the stem to its apex. AVherever 

 a branch arises [Fui. 3, i), its base is a prolongation from the distal side of the 

 expansion of the main stem, just below the calycles ; and the same relation obtains, 

 where a branchlet arises from a branch. When extended from its calycle, the 

 hydra {Fig. 2, c p t) has no thicker outer wall {a) than the main stem, but the 

 inner wall {h) is at least twice as tliick, whereas, in the stem, it scarcely more 

 than equals the outer one in this respect. When retracted within its calycle 

 {Fig. 2, m\ the outer wall («) is sometimes very thick, but at other times it is 

 comparatively thin {Fig. 3). The outer wall is not altogether free from the 

 chitinous sheath, which envelops the whole hydrarium, but here and there film-like 

 prolongations {Figs. 2, a', and 12, a') from the wall attach it to the inner face of 

 the sheath. In the younger stages of development, these processes are quite 

 numerous {Fig. 15, a'), but, with increasing age, the greater part of them are 

 retracted. In the reproductive calycles, not only the outer wall, but the inner 

 one also, enters into the composition of these props, and, moreover, the chymiferous 

 cavity pervades them to a considerable extent {Figs. 10 and 10% h). As reo-ards 

 the reproductive calycles, it is noteworthy that the diverticula arising from their 

 axis {Figs. 10, a\ and 10% /), originate from, or near the terminal portion {g) 

 of the axis, and branch more or less, as they project toward the base and down 

 the sides of the calycle. 



Proles mcdusoidea. — lt is only during the breeding season, from May to Sep- 

 tember, that the reproductive individuals {Figs. 7, 8, 9, 10, and lO'') are present. 

 A fully-developed calycle contains a prolongation of the stem of the hydrarium, 

 and a medusoid which buds from it; the first extends, as a uniform, double-walled 

 tubule («i c), from the base to the apex of the chitinous calycle, at the latter 

 point expanding into a disk {g), which completely fills the aperture; and the 

 second, the medusoid, a double-walled sac {h l?), with a central, single-walled, pro- 

 bosciform body (/), occupies nearly the whole available room within the calycle, 

 the axial prolongation («i c) of the hydrarium being pushed aside against the 

 chitinous investment. The two walls {b h^) of the medusoid are directly prolonged 

 from the tubular axis {a^ e), and are about as thick as those of the latter, and 

 totally devoid of radiating or circular chymiferous canals. The probosciform, axial 

 prolongation (/) of the actinostome is a duplicature of the inner wall {li^) of the 

 medusoid; arising from a point {l^) just within the junction of the latter with 

 the tubular axis, it forms a sac within a sac, and is the immediate investment of 

 the chymiferous cavity, which is prolonged from the stem into the medusoid. 

 VOL. IV. 42 



