T. B. ROSSETER ON THE GENITAL ORGANS OF TAENIA SINUOSA. 85 



in the structure of this particular platyhelniinth, and we have 

 only one other instance of it occurring in the Cestoidae — viz. 

 in Krabbe's Taenia frag His, which he took from Anas crecca. 

 No mention is made of this dark point by Rudolphi, in his 

 synopsis, and Zeder's drawings of the generative organs do 

 not contain it (Fig. 4). From the days of Dujardin and 

 Stiles no explanation has been given as to what part this 

 spinous body plays in the structural economy of this particular 

 helminth. The hispid (spinous ?) cirrus-sheath is of common 

 occurrence, and armed suckers occur in Davaniae cchinobothridae 

 and Echinocotylus rosseteri, each having their uses — the former 

 to assist in the act of coition, and the latter to anchor the 

 scolex in the mucous membrane of the intestine ; but this 

 globular spinous sac performs no such function. How, then, 

 are we to explain its presence, and what inferences are we to 

 draw as to its utility in the structural economy of tape-worms ? 

 In my opinion we must look backwards for a solution of 

 the problem. 



The more one studies and considers the class Cestoidae from 

 a taxonomist's point of view, the more one is brought face to 

 face with the question, " Have the Cestoidae advanced or retro- 

 graded in the law of evolution ? ' : I cannot think, as some 

 eminent histologists have thought and at the present day do 

 think, "that nature, having once made a step forward by the 

 -creation of cell tissue, reverts under circumstances inimical to 

 its environment to the original form." If we advocate and 

 concede this theory in the lower forms of life, we must admit 

 the same causation in the other orders of nature, for one law 

 governs the whole, and the law of adaptability, in no matter 

 what form, or under what circumstances, is but an empty 

 theory. For instance, Trichodina pediculus, considered essentially 

 an ectoparasite on the tentacles of the fresh-water polypes 

 Hydra fusca and H. viridis, and sometimes on the branchial 

 appendages of the larvae of Triton cristatus, loses none of its 

 ectoparasitic characteristics on becoming an endoparasite on 

 the urinary organs of the adult Triton cristatus, having migrated 

 there through the gill slits during the act of absorption of 



