OR OSSEOUS SYSTEM. 15 



in the whole mass ; and these depend not on the polypi 

 which are but secondary, and often deciduous parts, but on 

 the general fleshy substance of the body. 



IV. Acalepha. Although there are no solid skeletons in 

 any of the soft, gelatinous, free, and floating animals of this 

 class, we generally perceive some firmer cartilaginous por- 

 tions of the body which afford support to the organs of pro- 

 gressive motion or of prehension. There are crescentic car- 

 tilaginous lamine around the inferior central part of the 

 body in the medusae which give support to the contracting 

 fleshy overhanging mantle, and to the absorbent tubes pro- 

 longed from that part. There are firm superficial longitudi- 

 nal bands in most of the ciliograde acalepha for the support 

 of those minute vibratile fins by the motions of which they 

 are carried through the sea. From the feebleness of their 

 muscular system, and from their swimming habits, it is 

 obvious that the acalepha can only support the lightest forms 

 of the skeleton. 



In the velella limbosa, Fig. 6, 

 which floats on the surface of the 

 sea there is a thin flexible per- 

 pendicular crest, (Fig. 6. 2. a,) 

 which is covered with a thin 

 layer of the deep blue coloured 

 mantle, and which rests obliquely 

 on a horizontal stronger transpa- 

 rent flexible plate, (Fig. 6, 2 b.) 

 The thin perpendicular crest, 

 which rises above the water and serves as a sail, appears to 

 be composed of the same condensed glutinous or horny 

 substance which composes the skeletons of the keratophytes. 

 The horizontal plate is thicker, concave below, marked with 

 concentric lines of growth, and gives support to the deep 

 blue mantle above, to the delicate marginal tentacula, (seen 

 at b b, in both views of the velella,) to the numerous tubular 

 suckers, and to the stomach placed beneath this concave 

 horizontal plate. 



The porpita, which is another of these floating acalepha, 

 presents a similar thin plate, to this horizontal lamina 

 of the velella, for the support of the same parts, but of a 

 round form, of a white colour, and of a porous texture. 



