INTESTINAL WORMS. 167 



worms appears to be similar, and for the purpose of secretion; a 

 renal secretion, as VAN BENEDEN supposes *.] In the thorn-headed 

 worms there are two lateral canals situated beneath the skin that 

 run the whole length of the body. 



We have already remarked that special respiratory organs are 

 wanting. So far as any action occurs between the air of the 

 medium in which these animals live and their nutrient fluid, it must 

 be effected by means of the skin. But Entozoa live, for the most 

 part, in situations where the atmosphere exists in a condition very 

 impure and unfit for respiration ; or where no air at all can enter, 

 as in the liver, brain, kidney, &c. It is therefore probable that 

 they derive from the fluids absorbed from the animals in which they 

 live, the quantity of oxygen necessary for their life, and that they 

 experience the influence of this gas only mediately through the 

 animals in which they live 2 . 



With respect to propagation: no genital organs, as noticed 

 above, have been detected in Cystic worms. What many writers 

 have described as eggs in these worms are calcareous corpuscles 

 beneath the skin, which also occur in Tape-worms. Their multi- 

 plication is effected by gemmation. In Ccenurus there arise on the 

 bladder on which the worm, or that extremity of it that bears 

 the head, is seated, little buds which again develop other buds; in 

 Echinococcus new bladders are formed within the parent bladder, 

 like cells within cells, in which young Echinococci are developed 

 that continue hanging by a thread for a time, after the containing 

 envelope is ruptured, and then fall into the cavity of the parent 

 bladder 3 . In Cysticercus the mode of propagation is yet unknown. 

 In Trematoda there is found on the abdominal surface, generally 

 nearer to the anterior than the posterior extremity, an opening 

 common to the organs of both sexes. From this a penis, usually 

 named Cirrus*, can be evolved; near this part the vagina opens. 



1 VAN BENEDEN Lettre relative a I' Hist, des vers cesto'ides, Ann. des Sc. nat. 30 SeVie, 

 Zool. Vol. xvn. pp. 21 30. 



2 Comp. on the respiration of intestinal worms, RUDOLPHI Hist, nat. Entozoor. I. 

 pp. 239 244, and CLOQUET Anat. des vers intestins, pp. 42 44. 



3 MUELLER in the Jahresbericht for 1835, Archiv. s. cvn. cvni. ; V. SIEBOLD in 

 BURDACH'S Physiol. ste Auflage n. s. 183 185. 



4 See the fig. of Distoma hepaticum in MEHLIS, figs. 8, 9, n. In fig. 8 is seen 

 near the cirrus the opening of the vagina, through which a bristle has been passed to 

 distinguish it. 



