MUSEUM OF COMPARATIVE ZOOLOGY. 269 



Rhizophysa gracilis, sp. nov. 



Plate VI. Figs 1-6. 



One of the most interesting Physophores found by us is a new species of 

 Rhizophysa. 



It differs from R. filiformis, Lam., in the position of the sexual bodies and 

 the form of the tentacular knobs. Its differences from R. Eysenhardtii are 

 still greater. R. gracilis has two kinds of tentacular knobs, neither of which 

 are present in R. Eysenhardtii. I am unable to say whether my species is the 

 same as any of those described by Studer.* Rhizophysa has a long, thread- 

 like axis, destitute of swimming-bells, covering-scales, and tastern. At one 

 extremity there is a float which, in all species of the genus, is relatively larger 

 than in other Physophores, with the exception of the single genus Physalia. 

 The axis is very contractile, coiling up when touched at the base of the float, 

 and then leisurely extending itself to its normal length. The stem walls seem 

 to be extremety sensitive to the touch of a foreign body. 



The float (fig. 2) is similar to an enlargement of the stem at one extremity, 

 and is formed of two parts, an external float and an internal air-sac. The 

 walls of the float are composed of two layers, and have the shape of an elon- 

 gated sphere (prolate spheroid). The outer walls are dotted with small cells, 

 irregularly placed. These cells are characteristic of the species, or at least are 

 not found in R. filiformis, the common Rhizophysa from the Mediterranean. 

 At the apical pole of the float there is an opening through which the cavity of 

 the air-sac communicates with the surrounding medium. 



The air-sac has the general form of the float, but is much smaller, and hangs 

 inside this structure, being suspended from its upper pole. The cavity of the 

 air-sac is in free communication with the external water through the apical 

 orifice of the float. Around this opening there are large patches of dark 

 brown pigment. The walls of the lower hemisphere of the air-sac (g) are 

 thicker than those of the upper, and have a yellowish green color. From the 

 lower surface of the air-sac, confined to the hemisphere (g) with thickened 

 walls, there hang into the cavity of the float many finger-like pouches, which 

 are sometimes bifurcated at their extremities, In R. gracilis these appendages 

 to the air-sac are open at their distal ends, so that their cavities seem to freely 

 communicate with that of the float. On one side of the base of the float, pro- 

 jecting from its external walls, is an excrescence (<) filled witli small globules 

 whose function is unknown. f A similar appendage to the float has not boon 

 seen by me in R. filiformis. 



The polypites (p) of R. gracilis resemble closely in shape those of R. fili- 



* Zeit. f. Wiss. Zoob, XXXI. 



t A natural homology of this body is that it is an undeveloped polypite. The 

 transparent spheres within are probably babbles of air which have made their way 

 into the float through its apical opening. 



