DISTOMIDJ]. 23 



name will ever be imperisliably associated.* In all essential parti- 

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

 at tlie same time, sncli a multitude and variety of data bearing 

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

 utterly impossible — consistent with tlie design of tlie 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 EcJdnostoma (" 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 successfully 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, Yon Siebold, and others. 



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

 produces a limited 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 (&), the latter subse- 

 quently developing in its interior an oblong organism called a 

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

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

 that the ciliated embryos here drawn from 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 with 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. 0. 



