40 ANIMAL PAKASITES. 



Arrived in the stomach, they are entirely or partially digested, diffuse 

 their brood there, and sometimes even pass in a half-digested state 

 into the intestine. With this the functions of the proglottides come 

 to an end, even if this be not the case previously, on their escape 

 into the outer world and the dissemination of their eggs there. 

 That proglottides cannot pass the stomach of an animal without 

 being digested, has already been stated. But even if particular 

 proglottides, after bursting in the intestine of an animal, should 

 be capable of healing up again and living for a longer period, it 

 would certainly be impossible t^at such proglottides should again 

 produce brood as the genitnlia for the production of germs are in 

 course of retrogression. With the dissemination of the brood 

 their function ceases. 



II. The six-hooked brood, enclosed in separate egg-capsules. 



These are called eggs, although their developmental history differs 

 greatly from that of ordinary eggs. The embryos enclosed in 

 the egg-shells are globular, naked vesicles, unlike their parents ; 

 the smallest of them measure only 0*022 mill., and the largest as 

 much as 0*05 ; they are destitute of any organs, scarcely fur- 

 nished with an oil-drop, possess an epidermis with a double out- 

 line, and usually bear six, but in the Tetrarhynchi only four, very 

 small, microscopic booklets, on their anterior extremity, so that 

 they carry their destination, that of boring forward through the 

 tissues, as it were, written upon their foreheads. The bodies 

 of these small, but very dangerous vesicles, whether armed or 

 unarmed, are always capable of motion. The embryos which are 

 destined to migrate into cold-blooded animals are in general 

 larger, possess larger hooks, and exhibit tolerably-distinct move- 

 ments, even at the ordinary temperature of a room. Those des- 

 tined to migrate into warm-blooded animals are much smaller, 

 have smaller booklets, and only exhibit pretty-distinct movements 

 at an elevated temperature (at the temperature of the stomach). 

 We may easily suppose that according to the greater or less size 

 of the vesicle which is to move forward through the tissues the 

 boring apparatus of the vesicle will also be larger or smaller. 

 Perhaps, some day, we shall discover some small, constant dis- 

 tinctions in the embryonal booklets of the different species; but 

 in general these distinctions are hardly to be called essential, and 

 in the embryos with very small booklets they disappear entirely. 



