DISTOMID^. 27 



cercaria in presenting a double crown of hooks surrounding the 

 head, but several of the organs (marked b, c, e, and g) will be 

 seen to correspond with those displayed in fig. 7. According to 

 Van Beneden the cephalic hooks commence to make then- 

 appearance immediately after the larva encysts itself In this 

 condition the larva next becomes transferred to the intestine of 

 some higher animal, and in this situation it gradually acquires all 

 those organs, the possession of which will entitle it to be called a 

 sexually mature or adult distome. Soon after its passive immigra- 

 tion to the final host, traces of the reproductive organs make their 

 appearance ; certain portions of the parenchyma being set aside, as 

 it were, or differentiated so as to form the matrices of these com- 



I"ia. 8. — The Pupa, or tail-lees Cercaria (Proglottis, Van Ben.) escaping from its ruptured cyst. — 



Van Beneden. 



phcated organs. These phenomena may be clearly understood by an 

 examination of Van Beneden's representation (fig. 9, p. 28), in which 

 the following parts are readily discernible : — Commencing from 

 above, we have the mouth (a), the buccal or cephalic sucker (b), 

 the pharyngeal bulb (c), the oesophagus {d), the digestive coeca (e, e), 

 the coronal spines (/), the contractile vesicle (g), the aquiferous 

 system of vessels (h), the matrices of the yelk-forming glands {i, i), 

 and also a central mass of cellules Qc, I), from which all the other re- 

 productive organs will in due time be developed. In the adult condi- 

 tion {Bistoma militare) the head is abruptly truncate in fi^ont, the 

 posterior extremity pointed and narrow, and the upper third of 



