248 HYDROIDiE. Part IV. 



in the first medusa {Fig. 6, c^). Each of these secondary medusae goes on develop- 

 ing its tentacular appendage {Fig. 12, g), exactly as did the parent {e), and in 

 the same way secondary {c^) and tertiary (c^) hernia?, and so on, arise from the 

 first hernia; whilst the tentacle of the primary medusa {Fig. 12) elongates to twice, 

 and finally to thrice the length of the disk {Fig. 13, g^). In such a state of 

 development, there being no less than ten or a dozen medusas attached to the 

 base of its tentacle, the primary medusa soon drops from the head of the hydra,, 

 from which it has arisen, and enjoys a free life {Figs. 14, 14", 15, and 15*). In 

 this condition it is not unlike some of the Siphonophorse. Indeed, no one can 

 doubt that if such colonies of medusaj had been first observed in the ocean, in 

 their free condition, away from the hydroids from which they originate, they would 

 have been referred to the Siphonophoras, and not to the Hydroids. In a discussion 

 of the natural affinities of the Siphonophora?, the genus Hybocodon cannot fail to 

 appear as an important point of evidence of the close relationship which unites the 

 SiphonophorjB and the Hydroids proper. For my own part, I have no doubt that 

 the Siphonophorte belong to the order of the Hydroidai, in which they will be 

 subdivided into a number of distinct families. 



Histologg. — The cells of the outer wall (PL XXIIP. Fig. 10, h bb) are arranged 

 in two layers, which, together, are about one one thousandth (xoVt) '^^ ^^^ iwch 

 thick. The cell contents are perfectly homogeneous, and, although there is no trace 

 of granulation, the wall appears darker than the inner wall {d). The cells are 

 irregularly polygonal, and have very thin walls. The cells of the inner wall 

 {Figs. 10 and 11, d) form but a single continuous layer, and are elongate in the 

 direction of the length of the stem, having an irregular lozenge shajoe, when viewed 

 from their inner face. They are about one two thousandth (j^oVo) ^^ ^"^ i^^^^ 

 long, and from one third to one half as broad, and four fifths as thick as the 

 outer wall. Their contents, as well as those of the semi-partitions {g^ g^), are 

 perfectly hyaline, with the exception of a large, rather faint mesoblast {Fig. 11, g^). 

 The cells of the semi-partitions {g^ g*) are disposed in three or four layers. They 

 are usually broader than those of the inner wall proper, being about half as broad 

 as long, but about the same length as the latter. They vary considerably in 

 thickness, according to whether the semi-partitions project more or less into the 

 cavity of the stem. On each side of every semi-partition there is a collection 

 of cells {dd), of moderate size, in one, or two, or three irregular rows ; each cell 

 contains a large, irregular, pigmentary, orange-red mesoblast, which occupies from 

 one half to two thirds of its diameter. It is these mesoblasts which give the 

 orange-red hue to the whole stem. With the exception of the space occupied 

 by the pigment-bearing cells, the whole interior surface of the inner wall and semi- 

 partitions is covered by vibratile cilia, which are about as long as the thickness 



