ANIMAL PAEASITES. 41 



These smallest forms are not only similar in all the species in 

 which they occur, but all the six hooklets are also similar to 

 each other. In the smaller vesicles these hooklets are about 

 0'0095 millim. in length, and have a crescent of 0*0038; in the 

 larger ones they are about 0-020 millim. long, with a crescent of 

 0-009. They are placed in pairs (two pairs laterally, and one 

 pair in the middle), with all their points directed outwards ; 

 there are no muscles at their base, their movements are produced 

 by the displacement of the neighbouring parenchyma of the body, 

 and take place in the same direction with all the hooks, as may 

 be seen distinctly in the Teenies of birds. The embryos generally 

 have a small clear space between them and their envelope. 



The shells or envelopes of these embryos (egg-shells) are very 

 different in the different species ; they are larger or smaller, 1 

 simple or multiple, smooth or rough, clear or dark, round or oval, 

 or furnished with appendages, &c. The eggs of the Tcenice of the 

 human subject, like those of the others which pass through a 

 vesicular stage, are brown or yellow, thick and firm, rough exter- 

 nally, or, as was formerly said, beset with pits (foveoli). Leuckart, 

 however, shows that, according to Welcker's statements regarding 

 the microscopic relief- picture, we must admit the existence of 

 numerous, perpendicular, stiff bacilla or hairs which are separated 

 by confluent crevices or chinks. In those cases in which, as in 

 Tcenia solium, it has been supposed that a concentric stratification 

 and radiate arrangement is to be seen in the shells, an optical 

 illusion takes place. The supposed radii are strongly refractive 

 bacilla ; the concentric strata are the optical product of the 

 projection of particular bacilla ; nothing of them is seen in 

 isolated fragments of shells. The clefts between the bacilla 

 appear sometimes even to have penetrated through the inner 

 layer. The size of the bacilla varies according to the 

 species ; the shortest are those of T. Echinococcus. Both the 

 bacilla and the inner smooth layer of the shells are chitinous. 

 The former appear to be destined partly to give the shells greater 

 firmness and partly to increase the surface exposed to digestion. 



1 The smaller the eggs the more easily will they, on the one hand, escape the 

 dangers to which they are exposed in the outer world, and the forces which may act 

 upon them ; whilst, on the other hand, we may suppose that a greater number of them 

 may be destroyed without any injurious effect on the continuance of the species ; as also 

 that, for the latter reason, nature has endeavoured to produce the smallest, or, according 

 to the proportions of space, the most numerous eggs in the largest proglottides. 



