THE SPOROZOA 169 



haemocoele (blood-vessels or body-cavity), and sometimes the true 

 coelom. Here it continues to grow, absorbing nutriment from its 

 host, until it becomes a ripe, full-fed sporont. It then encysts, 

 with or without previous association with another of its kind, and 

 the process of spore-formation or sporogony commences. Sporo- 

 blasts are formed which usually secrete sporocysts and give rise 

 to spores, and within the spore-envelope the sporoplasm breaks up 

 into sporozoites, eight in number as a general rule. In a few rare 

 instances there is endogenous reproduction by schizogony, in 

 addition to the ordinary sporogony. 



As has been said above, the Gregarines were the earliest Sporozoa to 

 be observed and studied, on account of their large and conspicuous size. 

 The history of the group may be said to commence, for all practical 

 purposes, with the founding of the genus Gregarina by Dufour in 1828. 

 From that time onwards numerous observers, amongst whom Aimd 

 Schneider and L4ger deserve special mention, have added to our know- 

 ledge of the abundance of genera and species of these parasites, or 

 have studied the details of their life -history and development. 

 Nevertheless, it is only in the most recent times, practically in the 

 new-born twentieth century, that the facts concerning the conjugative 

 processes have become accurately known, largely in consequence of 

 renewed investigations upon them stimulated by the interesting dis- 

 coveries made in other orders of Sporozoa. 



Occurrence, Habitat, etc. The Gregarines are confined for the 

 most part to invertebrate hosts, and have never yet been found in 

 any craniate vertebrate. The great majority of them lead blame- 

 less lives in the interiors of various arthropods, to which the 

 segmented forms comprised in the sub-order Cephalina are almost 

 confined. The unsegmented forms are found commonly, however, 

 in other groups also, especially in echinoderms, in annelids, 

 including gephyrea and hirudinea, and in tunicata. A few have 

 been recorded from turbellaria, nemertines, and enteropneusta, 

 and a doubtful species is known from Amphioxus. In molluscs, 

 however, they are almost unknown, the single recorded instance 

 being a species from the body -cavity of Pterotrachea. Thus the 

 Gregarines are to a large extent the opposite to the Coccidia in the 

 matter of the hosts they affect, the arthropods alone being ground 

 common to the two orders. 



The infection of the host is probably effected in all cases by 

 way of the digestive tract, and the sporozoites, when liberated 

 there, proceed to attack the lining epithelium. In some cases 

 the sporozoite traverses the epithelium without stopping, passing 

 on into the haemocoele, as in Diplocystis of the cricket, or into 

 the coelom or one of its subdivisions, as in Mo'nocystis. In other 

 cases it remains attached to an epithelial cell only by a small 



