ACALEPH.E. 235 



to the curious observations of M. Yogt. Fig. 95 is a representation of 

 Physophora hydrostatica, after M. Yogt's memoir. We see that the 

 animal is composed of a slender vertical axis, terminating in an aerial 

 bladder, carrying laterally certain vesicles, known as swimming-balls, 

 which terminate in a bundle of whitish slender threads. 



The aerial bladder is brilliant and silvery, punctured with red spots. 

 The swimming-bladders are encased in a transparent and somewhat 

 cartilaginous capsule, which is continued into the common median 

 trunk, the latter being rose-coloured, hollow, and very contractile ; in 

 short, it presents very delicate muscular fibres, which expand them- 

 selves on the external fan of the capsule, and is closed on all sides. 



The swimming-bladders are of a glass-like transparency, and of a 

 firm, compact tissue. They are attached obliquely and alternately 

 upon a common axis, presenting an exterior curvature, a round 

 opening, furnished with a fine, muscular, and very contractile limb, 

 and arranged like the iris of the eye. Their power of resistance is 

 increased by certain horny hollow threads, which are in direct com- 

 munication with the cavity of the vertical trunk, and have their origin 

 in a common circular canal. 



"The animal," says Yogt, "is enabled to guide itself in any 

 direction by means of the swimming apparatus or air-bags. These, 

 on opening, are filled with water, which is again ejected in the 

 contractile movement, for their movements may be compared to that 

 of the umbrella of the Medusae. It is the violent expulsion of this 

 liquid which enables the animal to advance diagonally through the 

 water, a kind of motion which is the consequence of its organization ; 

 for where both rows of air-bags are working in the direction of the 

 axis of the trunk, the organism will incline to the side which works 

 most, but always in such a manner that the aerial vesicle will be 

 borne forward." 



In its lower parts the trunk expands, becomes flat, and winds 

 itself in a spiral. It is hollow, and encloses a transparent viscous 

 liquid, in which very small granules are observed, which appear to be 

 the result of digestion. This disk is attached to three different sorts 

 of appendages ; we shall first address ourselves to the tentacles. 



These form a crown or bundle of vermiform appendages, of a 

 reddish colour, over an inch in length, and which are kept continually 

 in motion : these are formed of a glass-like cartilaginous substance ; 



