On Flukes. 31 



The vessels thus named vary greatly in disposition, not only 

 among the flukes, but also in other orders of parasites. This 

 circumstance, along with other considerations, has given rise 

 to much discussion as to their nature and function. Into the 

 debate we do not now propose to enter, but may remark in 

 passiug, that there do not appear any very good grounds for 

 considering them equivalent to the true blood-vessels of other 

 invertebrated animals. In the present example, however, it 

 will not be denied that the vascular arrangements bear a very 

 striking resemblance to that of arteries or veins; and the 

 centrally-placed pouch (as shown in Fig. 1 of the accompanying 

 plate) might very easily be taken to represent the heart. This 

 large cavity gives origin to two primary trunks, which pass 

 forward along the inner sides of the digestive casca; in their 

 passage they send off secondary branches which divide and sub- 

 divide until we arrive at a series of minute capillary ramifications, 

 the latter, according to Blanchard, terminating in small oval- 

 shaped sacs or lacunse. The last-named organs are placed 

 immediately beneath the skin, and appear to have a special 

 connection with that structure, the nature of which will be 

 subsequently considered. Whatever may be the significance 

 of these lacunae — and on this point much might be said — all 

 will agree that the arrangement of the vessels in connection 

 with them is extremely beautiful. Hitherto no one has dis- 

 covered any external outlet to the central pouch; yet, in all 

 probability, such an opening exists. Several observers have 

 considered this water-vascular system as directly connected 

 with the organs of digestion ; but, in maintaining this opinion, 

 they are clearly erroneous. 



In an anatomical and physiological point of view, the study 

 of the reproductive system possesses high interest. Nearly all 

 the flukes are hermaphroditic, that is to say, each individual is 

 at one and the same time both male and female. In the large 

 Figure (1) the essential organs connected with this system are 

 only partially indicated. The central tortuous canal is the 

 so-called uterus; this, as shown in the dissection below 

 (Fig. 4), communicates with two rounded sacs, one in front 

 of the other; in this situation it also subsequently divides 

 into two tubular branches; these tubes pass right and 

 left, one to either side of the body, and curving upwards, 

 after the fashion displayed in the drawing, they branch out 

 into exquisitely delicate ramifications which terminate in 

 little grape-like bunches. In different kinds of flukes these 

 botryoidal structures display a variety of appearance, and 

 are collectively denominated the yelk-forming organs. The 

 germs of the future eggs are developed in a separate 

 glandular body called the ovary, which latter, in the present 



