DISTOMIDJi:. 23 



name will ever be imperishably associated.* In aU essential parti- 

 culars Steenstrup's statements and facts have been verified, wliilst, 

 at the same time, such a multitude and variety of data bearing 

 upon this question have since been placed on record, that it is 

 utterly impossible — consistent with the design of the present work 

 — ^to discuss the subject at any very great length. I can do no 

 more than select one good example to illustrate the phases of 

 development, through which certain of the distomes pass ; adding, 

 however. Dr. Pagenstecher's general conclusions respecting those 

 modifications of structure and habit found to occur in certain 

 larval types. 



The species selected for illustration is the Distoma militare of 

 Rudolphi, a form which I have elsewhere arranged under Dujardin's 

 sub-genus EcUnostoma ("Synopsis," Linn. Proceed., vol. v., Zool. 

 Div. p. 34), but which I shall now, to prevent confusion, call a true 

 distome. The entire cycle of the life-development of this species has 

 been successftdly investigated by Van Beneden, from whose re- 

 searches I gather most of the following facts ; many of which have, 

 notwithstanding, been also more or less clearly determined by 

 Pagenstecher, Steenstrup, Von Siebold, and others. 



In the adult, or sexually mature condition, the -Distoma militare 

 produces a Hmited number of oval- shaped eggs (fig. 5 a) which, 

 after the usual process of yelk-segmentation, give birth to an oval, 

 free swimming, and finely ciliated embryo (b), the latter subse- 

 quently developing in its interior an oblong organism called a 

 sporocyst {Scolex, Van Beneden). AU the distomes commence 

 life in this manner, but Van Beneden is not absolutely certain 

 that the ciliated embryos here drawn fi:'om his figures (b and c) 

 were produced by the ova of the particular distome now employed 

 for illustration. This is, however, a matter of trifling consequence, 



* I entirely coincide witli Dr. Carpenter in regarding the expression "alternate 

 generation'' as calculated to mislead. The term " generation" should only be applied to 

 the products of sexual congress, and not to the non-sexual progeny in common with it. 

 It would certainly be more correct to speak of the " law of alternate gemmation " or 

 agamogenesis. — T. S. C. 



