752 Comparative Animal Physiology 



feet on the reproductive activities of the animal, although we know too little 

 about their roles to attempt any generalizations. 



Invertebrates: Hormones and Secondary and Accessory Sex Characters. 

 Many species of invertebrate organisms representing many phyla possess a 

 sexual dimorphism which has long been suspected to owe its origin, in some 

 measure at least, to hormones comparable to the gonadal hormones of verte- 

 brates.^^- '•*^ Even to the present,' however, no incontrovertible evidence has 

 been advanced to prove this is so. Nevertheless, there are numerous reports 

 in the literature suggesting hormones to be playing roles in this regard. These 

 come from observations on (1) parallel effects of parasites on gonads 

 and sexual differentiations; (2) parallel cyclical changes in the degree of 

 gonadal activity and certain secondary sexual characters; (3) results of ir- 

 radiation of the gonads or of the whole organism with x-rays or radium 

 ravs; and (4) results of surgical extirpation of gonads. 



It has been observed after operative destruction that the normal regenera- 

 tion of the copulatory apparatus of planarians requires the presence of the 

 gonads, particularly the testes.^''''' ^''^ In earthworms which lack the normal 

 secondary sex character, the clitellum, it has been observed that their testes, 

 but not their ovaries, had been destroyed by the sporozoan, Monocystis. 

 Surgical extirpation of the segments containing the testes, but not the ovar- 

 ies, was reported by one investigator to be followed by production of a re- 

 duced clitellum or none at all,^" whereas similar surgical castration was 

 reported by another investigator to be without any influence on either de- 

 velpoment of the clitellum or the mating behavior. ^•'' ^^ Since differentia- 

 tion of the clitellum normally commences simultaneouslv with the begin- 

 ning of spermatogenesis, it has been suggested""^ that both of these phenom- 

 ena may be parallellv influenced bv an extragonadal hormonal factor, al- 

 though completion of development of the clitellum may be hormonally de- 

 termined by a chemical factor liberated by the ripe sperm cells. This lat- 

 ter hypothesis finds support irt the observation that removal from mature 

 Eisenia of the anterior twelve segments containina the seminal vesicles re- 

 suits in the rapid disappearance of the clitellum and its subsequent failure 

 to redifferentiate.'^" 



The periwinkle, Littorina, exhibits an annual cycle of development of 

 gonads and accessorv characters. Observations on specimens whose gonads 

 were considerably damaged by trematode larval stages revealed incomplete 

 differentiation of their accessory characters. The copulatory arm, or hecto- 

 cotylus, of cephalopods has been reported as failing to show its character- 

 istic dift'erentiation after surgical castration. ^^"' ^'*'^ 



There have been numerous observations on the influence of parasitic 

 castration of male decapod crustaceans upon the size and forms of the pleo- 

 pods, chelipcds, and abdomen which typicallv show distinct differences be- 

 tween the two sexes. The partial to complete castration which normallv re- 

 sults from parasitization by rhizocephalans (jiarasitic Cirripedia) or bopy- 

 rids (parasitic isopods) is commonly accompanied by the failure of these 

 portions of the body to assume their typical masculine form, the specimens 

 approaching the female form in their sccx)ndary sexual differentiation. 



The interpretations of these results of parasitic castration of male crus- 

 taceans in general fall into either one of two major categories: (1) those 



