78 REPORT— 1867. 



taclea ; they have all a very distinctly chambered axis, composed of a single series 

 of cells whose contiguous walls fonn the transverse ^partitions. Each chamber 

 contains a clear liomogeneous fluid, with a nucleus which is usually seated on the 

 centre of the partition wall, and imbedded in a mass of granular protoplasm, which 

 is frequently continued through the axis of the cell in the form of a filament. 



The chambered axis of the tentacle becomes slightly thicker towards the 

 proximal end, and is here continued into the substance of the umbrella, through 

 nearly the entire of whose thiclmess it runs. The terminal cell of the tentficle 

 root thus plunged into the gelatinous mass of the umbrella is much larger than any 

 of the others which form the axis of the tentacle. Like the other cells of the 

 axis, it frequently presents a nucleus on some part of its walls. 



The axis of the tentacle is surrounded by an ectodermal tube, composed appa- 

 rently of membraneless cells, and having great numbers of minute, curved thread- 

 cells immersed in it. Near the root of tlie tentacle its ectoderm is thickened into 

 a cushion-like swelling, which becomes continuous with the umbrella margin. 

 Between the ectoderm and the chambered core of the tentacle is a well-marked 

 layer of longitudinal muscular fibres. 



The tentacle is thus absolutely solid in its entire extent, presenting nowhere any 

 trace of an axile tube. There can be therefore no communication between it and 

 the circular canal, which accordingly simply passes over the subumbrellar side of 

 its root. 



The author had been unable to find any trace of a velum which, certainly at the 

 period of liberation, does not present a visible rudiment, though in certain positions 

 of the medusa the optical expression of the thickness of the umbrella produces a 

 deceptive appearance which may be mistaken for a narrow velum. 



It will be thus apparent that there are two points in which Ohelia contrasts most 

 strongly with the great majority of hydroid Medusre, namely, (1) the structure of 

 the tentacles, and their entire want of connexion with the gastrovascular system, 

 and (2) the non-do\"elopmeut of a velum. 



The condition of the tentacles in OhcIia is entirely that of those organs in the 

 very aben-ant genus Cunina, where they are also inserted into the substance of the 

 imaljrella by a root chambered like the rest of the tentacle. The tentacles of Ohelia, 

 too, just like those of Cvnina, are remarkable for their slight extensibility, their 

 motions consisting chiefly in a spasmodic jerking up and down. The umbrella 

 possesses but slight contractility, and the progression of the medusa would appear 

 to be chiefly effected by the iin-like action of the tentacles. The habitually 

 everted condition of the umbrella, which causes what is its inner surface in other 

 medus.ie to become here convex, and its outer surface to become concave, would 

 seem to be connected with the non-development of a velum. 



in. The Stnicture of the Lithocysts in the Medusa o/" Campanulaeia. 



In the medusa of Campmmlaria Johnstoni (a medusa referable to the deep- 

 belled section of Gegenbaiu-'s genns Uiicopc) the marginal bodies or "lithocysts" 

 are situated on a chord-like structure which runsroimd the margin of the umbrella, 

 and which presents a little oval enlargement at each oi tlie points where it sup- 

 ports a lithocyst. This chord-like portion has been noticed in other medusfe, and 

 has been regarded as a nerve-chord with ganglionic enlargements ; Ijut it is plainly 

 nothing more than tlie ectoderm of the lower smface of the marginal canal. The 

 lithocyst is immersed for a slight depth in the marginal enlargement which sup- 

 ports it, and which sends a very delicate extension of its substance over the whole 

 of its free surface ; it consists of a spherical, transparent, and stractureless vesicle, 

 the greater part of whose cavity is occupied by a soft pulp. In this pulp, which 

 has necessarily a spherical fonn corresponding to that of'the containing vesicle, 

 there is excavated at the distal pole, or that which is opposite to the basis of 

 attachment of the vesicle, a pit-like cavity, and within this cavity, but not entirely 

 filling it, is the spherical, higlily refracting concretion. In the spherical pulp 

 itself no trace of structure could be detected, but its sm-face is marked by twelve 

 or fifteen delicate stria?, which take a meridional course at exactly equal distances 

 from one another. At the distal polo they all terminate distinctly in the margin 

 of the pit-like excavation, and may be thence traced to within a short distance of 



