METAMOEPHOSES OF TETEAEHYNCHUS. 139 



teriorof the Scolex there is formed a Tetrarhynchus, by a process of 

 gemmiparous reproduction ; and from the surface of the latter a kind 

 of viscid secretion exudes, which becomes solid, and forms a sort of 

 sheath made up of concentric layers. 



(363.) In this state of development there is found a sheath formed of 

 several layers, in the interior of which is a Trematode worm (Amphi- 

 stoma rliopaloides, Ch. Le Blond) ; and in the interior of the latter may be 

 perceived a Tetrarhynchus, which moves about vivaciously as soon as its 

 prison is opened. This Tetrarhynchus has been regarded by naturalists 

 as a parasite inhabiting the Trematode worm ; M. Yan Beneden believes 

 it to be a moveable gemma (bourgeon mobile}. 



(364.) It is constantly, if not always, found in cysts formed at the 

 expense of the peritoneum, in a great number of sea-fish cod, trigla, 

 conger, &c. In the third state of its existence, the Tetrarhynchus is 

 free, but in all respects resembling that which was enclosed in the Tre- 

 matode worm ; in a short time, however, transverse lines become deve- 

 loped upon the posterior part of its body, segments are formed, and 

 it becomes Tcenioid. In this condition it has been named Bofhrioce- 

 phalus, or more recently Rhynchobothrius. It is found in the intestinal 

 canal of the skate, among the first turns of the spiral valve. 



(365.) In the fourth and last phase of its growth, it presents a more 

 simple structure, the perfect animal performing the part of a tube 

 destined to disseminate ova. In this condition it is nothing more than 

 the last segment of the Taenioid form detached, in fact a Proglottis. In 

 this condition it is found in the intestine of the skate, in company with 

 the Bothriocephali : this is the mature or adult animal, provided with 

 complete male and female sexual organs. 



(366.) The adult Entozoon (the Proglottis loaded with eggs) is eva- 

 cuated together with the fasces of the skate, and with its ova serves as 

 food to fishes of small dimensions. The ova are developed either in 

 the intestines or the intestinal caeca of the devourer, and if the fish 

 which contains them happens to be swallowed by another fish, the de- 

 velopment still proceeds in its alimentary canal, or the caeca thereunto 

 appended. When arrived at the condition of a complete Scolex, after 

 having perhaps passed through the stomachs of several fishes that have 

 successively devoured each other, it perforates the intestinal walls and 

 lodges itself in the peritoneum, in which situation it forms its sheath, 

 and produces in its interior the " moveable gemma," from which is pro- 

 duced a Tetrarhynchus. The fishes containing the latter form are swal- 

 lowed in turn by the voracious Rays and Sharks, and their flesh having 

 been dissolved in the stomachs of their devourers, the Tetrarhynchus 

 becomes free, and continues its growth in the intestines until the last 

 forms (Proglottis'} are complete, which alone are furnished with a sexual 

 apparatus. Thus, from the production of the egg to the completion of 

 the mature animal, these parasites are continually passing into the ali- 



