348 Prof. P. J. Van Beneden on the Intestinal Worms, 



the Trematode and Cestoid worms. The chief difference between 

 the two consists in the presence of an intestinal canal in the firsts 

 and its absence in the last. Both orders of worms arc herma- 

 phrodite, and the organs of propagation are greatly developed. 

 An internal impregnation of the eggs cannot occur, since there 

 is no immediate connexion between the male and female organs*. 

 A remarkable peculiarity deserves mention — that the eggs are 

 formed by two distinct organs, of which the one secretes the 

 germ-vesicles {germigene), the other the vitelline cells of the 

 egg {vitellogene) . We may here notice a slight historical inac- 

 curacy which our author commits when he asserts (p. 192) that 

 O. Schmidt in 1848 first discovered this formation of the germ- 

 vesicle and of the vitelline globule in distinct organs in the Tur- 

 bellarice rhabdocoelce. It had been noticed earlier, as I think, by 

 C. Th. V. Siebold in Trematodes, and suspected in Cestoids also, 

 before it was observed in Turbellaricef. The eggs of Trematodes 

 are sometimes moulded into a definite form in a special organ 

 named by our author ootype. 



That system of canals divided into branches and connected 

 with a contractile bladder at the posterior extremity of the body, 

 which was at one time regarded as a vascular system, at another 

 as a respiratory system, is conceived by V. Beneden to be for 

 the secretion of urine (urea ?). 



In those Trematodes which exhibit alternating stages of ge- 

 neration, the embryo, on leaving the egg, is surrounded by a 

 covering of cilia. In the Cestoids, Van Beneden has never ob- 

 served anything like this. Here, on the contrary, hooks are 

 observed, six in number, which have nothing in common with 

 those hooks which afterwards form a circle round the head in 

 the tape-worms ; they occur also in species of Teenies in which 

 this circle of hooks is wanting [Tcenice inermes). In the embryos 

 of Ligula and Bothriocephalus these hooks are absent ; and also 

 in Tetrarhynchus Van Beneden has sought for them in vain. 

 These hooks work like digging or boring instruments, of which 

 the embryo avails itself to perforate the w^alls and pass into the 

 cavity of the body in which it is about to afiix itself as a para- 

 site. Van Beneden gives a very perspicuous account of the 

 action of these organs. Two of them are placed in the 

 middle, and push or glide forwards and backwards, working 

 like the snout of the mole as it digs in the ground ; the two 

 pairs of lateral hooks move outwards and inwards to make a way 

 through the tissues, as the fore feet of the mole push the loosened 

 earth aside. The vesicle of the Cysticerci is nothing else than 

 the body of the embryo of the Cestoids. Stein was the first to 



* Contrary to the assertion of C. Th. v. Siebold. 

 t See MiiUer's Archiv, 1836, pp. 235, 236. 



