84 PEACTICAL PABASITOLOGY 



closely to the Gregarines. They differ from both r Coccidia and 

 Gregarines, however, in that the alternation of generation, which is 

 an essential feature in the life-history of all three orders, is, in the 

 case of the Aggregata, associated with a change of host (from the 

 cuttle-fish to the short-tailed crab). The relationship between the 

 stages found in the two different hosts has only recently been estab- 

 lished. The parasites of the crab were formerly known as Aggregata 

 and were classed with the Gregarines, but the stages found in the 

 cuttle-fish were mistaken for Coccidia and received the name of 

 Eucoccidium (or Benedenia). Later authorities have preferred to 

 class the Aggregata with the Gregarines, but in view of their dis- 

 similar life-history it is doubtful whether this method of classification 

 is justified. 



Crabs (Carcinas mcenas, Portunus depurator, P. corrugatus, 

 Pinnotheres pisum, Eupagurus prideauxi, &c.) become infected by 

 the ingestion of cysts which have been excreted by cuttle-fish, and 

 which release their sporozites under the influence of the gastric 

 secretion of the crab. The sporozoites then penetrate the intestinal 

 wall and come to rest in the sub-epithelial layer, where they grow 

 rapidly larger. They form wart-like swellings upon the outer layer 

 of the gut, similar to those produced by the malaria parasite in the 

 gut of the mosquito. Their subsequent multiplication also somewhat 

 resembles that of the malaria parasite. Numberless daughter-nuclei 

 are formed by repeated mitotic division of the nucleus, and, at the 

 same time, the plasm splits up into several largish portions. The 

 daughter-nuclei move to the surface of these plasmic bodies, and there 

 surround themselves with a small quantity of the plasma. They are 

 now " new cells " or merozoites, and, like the sporozoites of the malaria 

 parasite, they release themselves by violent longitudinal jerks from the 

 plasmic masses, which remain as residual bodies. 



The further development of these merozoites takes place within the 

 gut of the cuttle-fish (Octopus or Sepia), by which the crab harbouring 

 the parasites is devoured. Here, also, the merozoites bore their way 

 into the intestinal wall, where they usually come to rest in a cell of 

 the submucosa, and where they develop into the large immotile 

 gametocytes. The female gametocyte 1 is richer in reserve material 

 than the male. It is also, as a rule, much larger, the diameter in 

 Aggregata legeri, from Octopus, measuring 0'25 to 0'3 mm. The 

 nucleus is strikingly large and may measure as much as O'l mm. 

 in diameter. 



1 As the outcome of Siedlecki's experiments they have always, up till now, been 

 regarded as the macrogametes of the so-called Eucoccidium. The organism was 

 believed to encyst, the presence of a membrane-like attachment from the host-cell 

 lending support to the supposition. 



