XXV111 
THE VOYAGE OE H.M.S. CHALLENGER. 
it forms the principal thickness of this membrane. Its fibres take a circular course, 
parallel to the margin of the umbrella. 
The manubrium presents the ordinary typical structure of the Hydroid body, its walls 
being composed of the two cell-layers, ectoderm and endodcrm, with an intervening 
structureless membrane, the mesosarc ; on the ectodermal side of the mesosarc lies a layer 
of longitudinal muscle fibres. 
The nervous system attains in the planoblast a stage of development more highly 
advanced than that presented by this system in the trophosome. In the planoblast it 
consists in its principal portion of two chords which run round the margin of the 
umbrella, one on the upper, and the other on the lower, side of the line of insertion of the 
velum. They are composed of fibres and ganglion cells, and are overlaid by a sense- 
epithelium whose cells carry sense-hairs. The organs of sense are situated on the 
margin of the umbrella and are chiefly of two kinds ; of these one consists mainly 
of accumulations of pigment cells. These are placed each at the base of a marginal 
tentacle, and are entirely confined to the ectoderm. They occasionally enclose a clear 
refringent spherule. To this form of sense organ the name of ocellus has been given. 
The other consists of transparent vesicles, within which are one or more cells with 
calcareous concretions or “ otolites.” They also enclose so-called “ auditory cells,” whose 
hairs surround the otolite cells. They are seated in variable number immediately over 
the marginal nerve-ring, and on the portions of the margin which lie between the bases 
of the tentacles . 1 They are known as otocysts. The otocysts like the ocelli are entirely 
ectodermal, and must be regarded as a special differentiation of the epithelium which 
covers the nerve-ring. 
The generative elements are developed either in the walls of the manubrium or along 
the course of the radiating canals. Those planoblasts in which the generative elements 
are found in the walls of the manubrium belong to the section Anthomedusae and have the 
marginal sense organs, when differentiated, in the form of ocelli, while those in which the 
genitalia are borne in some part of the course of the radiating canals belong to the section 
Leptomedusae, and have the sense organs in the form either of otocysts or of ocelli . 2 
1 In Obelia the otocyst is in contact with the base of the tentacle, but even here it does not lie in the meridional 
line of this, but is placed laterally, and thus really lies in the inter-tentacular spaces of the umbrella margin. The 
Ohelia planoblast is further exceptional in having its umbrella so shallow as to be almost disc-shaped, in the velum 
being rudimental, and in the marginal tentacles having their roots plunged into the substance of the gelatinous 
umbrella, being comparatively rigid, and having their axis occupied by a solid endodermal core. In the characters 
thus presented by the tentacles Obelia shows an obvious approach to the Trachomedusae and Narcomedusae.. 
2 1 adopt these names as proposed by Haeckel for the two groups indicated by them, in preference to the names, 
Ocellatae ( = Anthomedusae), and Vesiculatae ( = Leptomedusse), by which these groups have also been designated, the 
characters expressed by the latter names not being always applicable to the Medusae to which they are intended to 
apply. Haeckel divides the Craspedotae into two primary sections, the Leptolinae and the Trachylinae, the former being 
further divided into the Anthomedusae and the Leptomedusae, and the latter into the Trachomedusae and the Narco- 
medusae. The Anthomedusae among the Leptolinae, and the Narcomedusae among the Trachylinae, are distinguished by 
having their gonads in the walls of the manubrium ; the Leptomedusae among the Leptolinae, and the Trachomedusae 
among the Trachylinae, by having them in the course of the radiating canals. 
