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let of the sperm-sac is by a thick, protrusile, and retractile penis. 

 In those species which possess a foot, the intromittent organ is 

 soldered to its dorsal side, and is often so greatly developed that the 

 foot itself appears as an appendage. The penis is protruded by 

 eversion ; and is then seen to be a thick column with the extremity 

 truncate and ciliated. The sexual coitus has been witnessed by the 

 author in several instances. 



For a parallel to the curious facts thus established, the author 

 considers we must look to the Crustacea. The Hectocotylus of 

 certain Mollusca is scarcely an analogous case ; nor are those 

 Entozoa in which the males are organically united to the females. 



In the Crustacea, however, many examples occur of a sexual 

 difference which may be compared with that of the subjects of this 

 memoir. In the genera Bopyrus, Phryxus, and lone, the males are 

 notably smaller than the females, very diverse in form, and in some 

 respects inferior in structure. In the Siphonostoma, " the males are 

 extremely small, and do not in the least resemble the females" 

 (Baird) ; though those of different genera bear a strong resemblance 

 inter se, even where the females are very dissimilar. So low is their 

 grade of organization, that Burmeister has attempted to prove the 

 minute males to be embryonic forms. Finally, in the Cirripedia, 

 Mr. Darwin has proved the existence of males in the genera Ibla and 

 Scalpellum, which are very minute as compared with their females, 

 excessively abnormal in form, and in some respects in an embryonic 

 condition, though unquestionably mature, as shown by their sperma- 

 tozoa. And, what is still more interesting, there is, in these male 

 Cirripedia, "no vestige of a mouth, or masticatory organs, or 

 stomach." The same observer describes the internal structure as 

 "a pulpy mass with numerous oil-globules;" and the sperm- vesicle 

 as " a pear-shaped bag at the very bottom of the sack-formed animal 

 containing either pulpy matter, or a great mass of spermatozoa," 

 terms which might have been employed in describing some of the 

 male Brachioni. 



In all these analogies, the author finds additional reasons for 

 assigning to the Rotifera a zoological rank among the Articulata. 



