THE RADIATE CHARACTER. 139 by the means of these muscles that the extrusion of the polype takes place : these muscle-bands, drawing in the membrane to a concave form, diminish the con- tained space, which is already full, either with water, or as I rather suppose with the vital juices ; the only yielding part is the long body of the polype, which accordingly is forced out through the proper aperture. It is manifest that the radiate structure is becoming a subordinate character in these zoophytes ; at least so far as that character implies a perfect circular symmetry. This Eucratea for example has certainly a dorsal aspect and a ventral one : the direction of the intestinal canal, and the position of the excretory orifice making sufficiently plain which is the former. For from this arrangement, which is almost exactly repeated in some of the tubicolous Rotifera, as CEcistes for example, the oval orifice is gradually brought lower down the back by successive stages in Melicerta, Limnias, and Stej)ha7ioceros ; until in Mo- nocerca, Farcularia, and Notommata among the illoricate Rotifera, it attains the normal situation which it holds in the higher animals. Hence I have not scrupled to c^ll this the dorsal side of the zoo- phyte, in the preceding description. While on thfe subject I may mention that Eucratea frequently inflates the membranous integument just below the anus, in a manner common to many of the Rotifera. The ciliary action is doubtless in some measure involuntary ; but the tentacles have the power of separate and voluntary movement. I observed an