Endocrine Mechanisms 753 



which assume that the presence of the parasite has influenced the expression 

 of the genetic mechanism of sex determination, and (2) those which assume 

 that the testes or some other tissue produced an endocrine possessing andro- 

 genic activity. In support of the first of these two types of explanations is 

 the observation that certain decapod crustaceans normally possess hermaph- 

 roditic gonads, or are readily induced to develop them under the influence 

 of parasitization. Some species, such as Leander, which show no secondary 

 sexual character changes on castration may be thought to possess a relatively 

 stable mechanism of sex determination, not easily influenced by the parasite,"*" 

 whereas others like Ufogehia possess very labile mechanisms and conse- 

 quently show considerable feminization after parasitic infestation. A num- 

 ber of investigators have noticed that the higher fat content of the blood and 

 liver characteristic of normal females is also often observed in parasitized 

 males.**^ It is postulated by these investigators that the parasite imposes 

 much the same metabolic demands on the host as are normally made by the 

 developing eggs of the ovary, and that associated with the increased fat 

 metabolism is the production of a "sexual formative" stuff which parallelly 

 influences the development of both the gonads and the secondary Sv x char- 

 acters. 



The second type of explanation of the observed influences of parasitic 

 castration in males assumes the operation of a masculinizing hormone. Ac- 

 cording to this hypothesis the animal after parasitization does not become 

 feminized but assumes a neutral form, which chances to resemble more 

 clearly the female than the male. Considerable support is given this view 

 in observations on the influence of three species of parasites on the crab, 

 Munida sarsi. Two smxaller parasitic crustaceans, Triangidus munidae and 

 Lernaeodisciis ingolfi, totally or partially castrate the crab and produce strik- 

 ing modifications in the male secondary sex characters; a much larger para- 

 site, Trianotdtis hoschviai, leaves the gonads functional and does not modify 

 the sex characters. Such observations appear to exclude in these instances a 

 direct influence of the metabolic demands of the parasite in inducing the ob- 

 served changes. Other investigators, however, have failed to find a correla- 

 tion between the degree of gonadal atrophy and the extent of suppression 

 of the male characters and suggest that a tissue other than the testis pro- 

 duces the hormone in question. 



Another kind of observation suggesting an action of a hormone in male 

 decapod crustaceans is the seasonal cycle of changes in the copulatory ap- 

 pendages of the crayfish. These appendages assume a sexually functional 

 form (form I) at the time of the late summer molt. This is a time when the 

 testes are large and active. At the time of the spring molt, a period of low 

 gonadal activitv. they revert to a non-functional condition (form II). Ex- 

 perimental induction of molt during the winter months when testis activ- 

 ity is similarly low always produces form 11.^^^ These observations may be 

 equally well explained in terms either of the influence of a gonodal hormone 

 on the appendages or of the parallel action of a hormone from some other 

 source in the body on both gonad and appendages. 



Parasitic castration of female crustaceans is in general accompanied by 

 little or no change in the general form of the body and appendages. It has 

 been observed, however, that the brood pouch of Aselhis and that of Dafhnia 



