MONOSTOMIDiE. 19 



marked excretory or water-vascular system exists, but no ner- 

 vous elements liave yet been described in any member of the 

 family. There can be httle doubt, however, that if carefully sought 

 for, a pair of cephalic gangha and two lateral cords will be 

 discovered. 



Development. — ^Most of the Monostomes reproduce oviparously 

 in the ordinary manner, but in some instances the mode of repro- 

 duction is either ovo-viviparous, or altogether viviparous. This 

 singular phenomenon occurs at least in one species (Monostoma 

 mutahile), and its discovery by Von Siebold (in 1835) has tended 

 to throw much light upon the genesis of flukes in general. Van 

 Beneden and others have also ably followed up this subject, and 

 have added many important particulars. The facts, as these 

 authors have described them in the above-named species, may be 

 briefly stated as foUows : — Whilst still within the uterine tube the 

 ova develop in their interior living embryos, which latter, at an 

 early stage, display a pair of eye-specks, and, at the time of 

 their exclusion from the egg-shell (chorion), have their bodies 

 entirely clothed with vibratile ciha. At the same time, by a 

 process of internal budding, the cihated embryo has developed in 

 its interior a larva, differing in form from the ciliated embryo 

 (nurse or proscolex of Van Beneden) as much as the latter itself 

 differs from the original parent. The ciliated animalcule (embryo) 

 swimming freely, assumes for a time an independent existence, but 

 after a while gives birth to the contained growing larva (scolex of 

 Van Beneden), which latter itself becomes converted into a 

 secondary nurse or sporocyst, developing in its interior several 

 larval flukes in the condition of the well-known cercarige. The 

 actual conversion of the cercarige into the sexually mature monos- 

 tome, has not, so far as I am aware, been observed in any member 

 of this particular family ; but, from what we know of the final 

 stages of the process as witnessed in the Distomata, there can be no 

 doubt that these larval great-grandchildren, or cercarige, ultimately 

 become transformed into adult examples of Monostoma mutahih^ 



