138 ENTOZOA. 



sexually mature worm is one or other of the various species of 

 tetrarliyucli already described by systematists as infesting the 

 larger sharks. Be that as it may, we have here a remarkable 

 illustration of the power of the scolex to form the tsenioid con- 

 dition until it has reached a length of two or more feet; at 

 the same time, it is wandering about the viscera and muscles 

 of its intermediate piscine host, growing rapidly and developing 

 well-marked joints, without acquiring, apparently, the slightest 

 indication of sexual organs. When these parasites are numerous, 

 — as is commonly the case, and I have seen scores of such scolices 

 in the liver of a single fish, — it becomes extremely difficult to 

 unravel their tortuous windings; for not only is their course 

 often very erratic, especially in the liver, but they twist round one 

 another so as to form a complicated network of cords. In the case 

 of the young fish, here figured, the parasites were comparatively 

 few in number, those from the muscles being more readily dissected 

 out. On being removed several were dropt into a tumbler of sea- 

 water, when, to my astonishment, they moved about actively ; I 

 say " astonishment," because the fish had been dead nearly an 

 entire week, and had been cast aside, as refuse, by the fish- 

 salesman of whom it was purchased. On being deprived of their 

 investing capsules, the proboscides were protruded and retracted 

 in an irregular alternating manner. These movements continued 

 for three days. The longest specimens of the parasite measured 

 about twenty inches, but I had previously met with examples which 

 were probably double that length. 



Whilst still within their capsular coverings, these tetrarhynchs 

 display a globular projection at their anterior extremity (Fig. 2). 

 This appearance at first conveys the notion of a head, followed by a 

 narrow neck ; in reality, however, the little ball includes not only the 

 head and neck, but a remarkable sub-cervical enlargement which is 

 interposed between the so-called neck and the upper division of the 

 body. These parts are readily seen when the animal is hberated 

 (Fig. o). The head itself, when viewed from above, displays a 



