OF THE CYSTIC ENTOZOA. 87 



already described as takifig place in Aceplialocystis. There ap- 

 pears to be two modes of generation, namely, one for the enlarge- 

 ment of the original group, and another for the formation of new 

 groups in other parts of the peritoneum. The first of these 

 modes proceeds in the Astoma, from the animal becoming so dis- 

 tended, in consequence of the increased size and number of the 

 young within it, that it bursts when the young are exposed, and 

 the parent sac, which is now useless, absorbed, the progeny in 

 the meantime becoming attached to the peritoneum.* The ex- 

 ternal membranes in Diskostoma spread over the as yet uninfested 

 portions of the peritoneum, and give origin to a number of cells 

 from the attached surface, each of which, becoming parents, gra- 

 dually increase in size, from the addition of new matter within the 

 young cells. These young cells are the germs of the future animals. 

 The other mode of developernent or that intended for the forma- 

 tion of new groups is similar in both animals. The young or se- 

 condary cells, bursting from their formative cell, by some means 

 escape from the parent sac, and so gain a situation at some dis- 

 tance from the original group, where they become attached, in 

 time throw off young cells, and thus become the origin of a new 

 set. 



Relative to the mode of reproduction in these animals, it is 

 found that in Astoma, and the higher cystic entozoa, the numbers 

 proceeding from one parent may be unlimited, whereas in ace- 

 phalocystis generation ceases with the quaternary series of young, 

 unless this series, or the gemmules of some of the preceding, 

 escape from the original sac, and are able to form a nidus in any 

 portion of the liver, or other organ yet uninfested. For it ap- 

 pears necessary to the existence of the common hydatid that it 

 be completely enveloped in the tissues of the infested being. To 

 ensure this normal habitat, then, the animal must escape during 

 the period of its gemmule existence from the parent; but, as 

 most generally happens, if the parent hydatid be so deeply buried 

 as not to allow free rupture of its coats within a certain period, 

 decomposition ensues as already described, and so existence is 

 terminated ; if, on the contrary, the parent hydatid be so near 



* See Preparation in Museum of Royal College of Surgeons, Edinburgh, No. 2244. 



