Chap. IV. HYBOCODON PROLIFER. 



245 



however, very different from those of the latter genus, since they have no common 

 peduncular axis of attachment. In Hybocodon the first medusa {Fig. 13) arises 

 directly from the actinal area of the disk (« h), Avhile from the marginal termination 

 {h') of one of the radiating tubes (h^) of this medusa numerous similar medus* 

 are developed, the latter again giving rise to other medusae (c^ c" h P t% in the 

 same manner, and from a corresponding place on their margin. In this 'way is 

 produced a branching axis, which extends laterally as well as longitudinally, 

 and, in the full breeding season, crowds the disk with its burden of medusJ 

 (Fiff. 2, d d' d^ e). 



The head of the hydroid, as we have before said, is joined to the stem by 

 a constricted neck, which is capable of great distension, and when in this state 

 {Fiff. 3) the internal longitudinal rows (a) of orange-red pigment are very easily 

 seen through the walls. But it is not the pigment lines alone which give rise 

 to the ridged appearance of the stem, for upon making a transverse section of 

 the walls (PI. XXIIP. Fig. 10) we find, that between every double pigment row 

 {Fiffs. 10 and 11, dd) there is a ridge (^*), or semi-partition, very similar to that 

 described in Parypha (PI. XXIIP. Fig. 7, /). It is transversely, broadly triangular, 

 and projects more or less into the cavity of the stem, but the combined ridc^es 

 never form a solid central core, as in Tubularia. In the oldest hydroids we have 

 counted sixteen of these ridges at the upper part of the stem, but, passing down- 

 wards, they merge into each other, as in Parypha and Tubularia. The inner wall 

 {Fig. 10, d), upon which the semi-partitions are based, is composed of a single 

 layer of cells, and is about ^i^ of an inch thick, or four fifths of the thickness^'of 

 the outer wall {l> hb). The whole interior surface, excepting where it is covered 

 by the pigment cells {dd), is lined by vibratile cilia. The outer wall consists of 

 a double layer of cells {h hh), and is ^J^^ of an inch thick. Both of these walls 

 are so transparent that it is possible to see the mesoblasts {Fig. 11, g^) of the 

 cells of the semi-partitions through the latter. 



Proles medusoidea. — Thcve is a close resemblance between the free medusa of 

 this genus (PI. XXV. Figs. 14, 14^ 15, and 15^), at the time it drops from ' the 

 parent, and that of Coryne mirabilis at a corresponding age (PI. XVIII. Fig. 15M 

 except in the number of their tentacles; in fact, a medusa of Hybocodon may 

 be said to be a Coryne with only a single tentacle. Nor does the fact that the 

 former produces medusae from the base of its tentacle invalidate the comparison, for 

 some of the Sarsiae do the same. The proportions of the disk, or bell-shaped 

 umbrella, its size, the relations of the outermost, the middle, and the innermost 

 walls, the radiating and circular canals, the two walls of the proboscis, and the 

 three walls of the transverse septum, are the same as in the medusa of Coryne, 

 to which we refer the reader for full details of the structure of these parts. The 



