144 C. F. ROUSSELET ON TRIARTHRA BRACHIATA. 



the shoulders, and seem hollow, but are not actuated by sj)ecial 

 muscles. When swimming the two lateral spines lie against the 

 sides of the body, and the posterior spine is trailed behind ; but 

 when the head is retracted they all three spring out at right 

 angles to the body — merely, as I think, by the pull exercised on 

 the integument and by the pressure of the internal fluid. The 

 posterior spine, it is then seen, is not quite terminal in position, 

 but stands out on the ventral side a little above the base. 



The shape of the body is cylindrical, slightly more than twice 

 as long as broad, and truncate in front. The ciliary wreath is 

 nearly circular, with a projecting lip on the ventral side. Two 

 red eyes, set close together near the anterior surface of the head, 

 are present. The rest of the anatomy is quite normal, and calls 

 for no remark. 



The male has not yet been seen, but a fertilised resting Qg^ of 

 characteristic Triarthra structure was seen, having a cellular 

 annulus all round the longer axis (fig. 8). 



Mr. F. E. Dixon-JSTuttall has been good enough to draw fig. 7 

 for me from a mounted specimen, which, he thinks, is not so 

 satisfactory as if he could have seen the living animal. 



Size of body alone : 95 /x = ~-j in., of spines 65 /^ = ~j^ in., 

 total length, including posterior spine: 156 /x = y^^^ in., found 

 singly on Putney Common and in water received from the north 

 of Ireland ; rare. 



A few remarks on the spines of Triarthra in general will not 

 be out of place here. These more or less long, chitinous and stiff 

 appendages have been called "skipping spines" by the various 

 authors. I have not, however, observed that any real skipping or 

 displacement is eflfected in these rotifers — such as undoubtedly 

 takes place in Pohjarthra — when the head is retracted and the 

 spines fly out. These spines, it seems to me, are protective in 

 quite a different way. I have often seen an Asplanchna trying 

 to swallow Triarthra longiseta by suddenly dilating its pharynx 

 and thus producing a sucking action, when Triarthra usually 

 saves itself by promptly spreading out its long spines. Some- 

 times, however, the attack is renewed from an unexpected 

 quarter, and occasionally Asplanchna succeeds in sucking in its 



