Miscellaneous. 143 



the ^^ginid(B and Trachynemidce, which present this structure, are 

 rigid. 



The contractile tentacles owe their contractility to a muscular layer 

 situated between the cellular axis and the external epithelium. This 

 cellular axis is only a dependence of the internal epithelium which 

 lines the digestive cavity (llydroids) or the marginal canal (Medusae). 

 It probably acts as an elastic organ antagonistic to the muscular 

 layer. 



The second kind of connective tissue is a substance destitute of 

 cells, which forms the umbrella of all the simple Medusae, including 

 the gelatinous substance of the natatory bells and covering laminae 

 of the Siphonophora. Sometimes this substance is entirely homo- 

 geneous ; sometimes it is traversed by numerous fibres very like the 

 elastic fibres. In an ^^quorea these fibres are attached to a mem- 

 brane capable of isolation, placed beneath the epithelium of the 

 convex surface of the umbrella. 



The third form is the well-known gelatinous substance with dis- 

 seminated cells of the umbrella of the higher Medusae. Professor 

 Kolliker agrees with Professor Virchow in denying the existence of 

 these cells in Cyanea capillata. — Bibl. Univ. May 1865, Bull. 

 Scient. p. 66. 



On a New Type in the Group of Ascidians — Chevreulius callensis. 

 By M. Lacaze-Duthiers. 



After describing the general characteristics of the Ascidia, the 

 author says : — The specimen which forms the subject of the present 

 memoir exhibits an exceptional and very remarkable arrangement, 

 which masks the true characters of the group. All the indivi- 

 duals of the genus Chrevreulius are without stolons or buds which 

 might lead to their being approximated to the social Ascidia, and 

 still less to the compound forms. Their form is that of a cylinder, 

 free at one extremity, adherent by the other, and slightly flattened 

 ou that side which is in contiguity to the foreign body. The free 

 superior extremity presents the characters of the genus. 



The test, which is of a yellowish colour and cartilaginous, is suffi- 

 ciently resistant to retain its form after desiccation ; its thickness is 

 not great, and it resembles a thin lamina of pale horn. When it is 

 contracted, the orifices are not visible ; but as it becomes distended, 

 more than half of the flat upper extremity of the cylinder is soon 

 seen to detach itself towards the circumference, and to rise by moving 

 as if round a hinge placed on the side of the cylinder which is 

 flattened. 



Beneath the plate which rises thus so as to form a right angle 

 with its former position, and which represents a valve, there appears 

 a white transparent tissue — a membrane stretched from one side to 

 the other of the separated parts, so as to fill up the great fissure 

 produced by this sort of gaping. Upon this membrane two mamillse 

 soon rise, at the summit of which open the two orifices characteristic 

 of the Ascidia. One of these leads into the branchial chamber, and 



