There have been certain organic remains* figured and described as belonging to the Tunicate family, but which (with the exception of the obscure and indeterminate Ischadites liwnigii) have been found to belong to the family of the Cystidea of Von Buch, closely allied to the crinoidean family of the Radiate.
At first sight there is considerable resem blance between many of these cystidean forms and the ascidian genus Boltenia, the body being globose or subcylindrical and pedun culated. In the Cystidea there are two more or less terminal orifices, and a third lateral aperture. The whole animal is coated with hex agonal plates variously ornamented ; the stem, perforated throughout and giving evidence of quinary arrangement. " The mouth," says Von Buch, " is planted in the central part of the upper surface, generally in a moveable proboscis, covered with minute plates ; the anal orifice is small, close to the mouth, per forating a plate, not surrounded by separate valvules ; and the third aperture, probably the ovarial orifice of the animal, is placed further towards the middle, but almost inva riably on the upper half of the body on which the mouth is placed. It is round or oval in
form, not connected with the mouth, and often covered by a five or six-sided pyramid, which seems to be composed of as many little valves." We have introduced this description that we may here point out the general similarity of the external form of these obsolete radiate animals to sonic of the ascidian group, and that m e may in particular point out the very similar armature of ornamented hexagonal plates present in Chelyosonia, with its valve surrounded orifices. With regard to the ad ditional orifices, we have but to lengthen out the oviduct or efferent vessel of the Aacidia, and continue it to the surface, as in the mal formation noticed and drawn by John Hunter, and a very similar arrangement of parts will apparently exist.
There are several points of analogy between some forms of Ascidice and of Zoophyte asci doida (Bowerbankia, &c.) and the Radiate: into this subject we must not now enter ; we can only allude to the observations of Messrs. Forbes and Goodsir on the Pelonaia (see p. 1239.), and leave the subject open to further URETHRA.—(Lat. Urethra ; Gr. olyilOpa; Fr. Urithre.)