326 THE PROTOZOA 



are, however, in more common use than the other names of the 

 subclasses given above.* 



The subclass Telosporidia, as mentioned above, includes the 

 three orders Gregarinoidea, Coccidiidea, and Hsemosporidia. 



ORDER I. GREGARINOIDEA. 



The chief characteristics of this order are First, that the tropho- 

 zoites are parasites of epithelial cells in the earlier stages of their 

 growth, but in later stages they become entirely free from the cells, 

 and lie in cavities of the body ; their most frequent habitat is the 

 digestive tract, but sometimes they are found in the body-cavity or 

 the haemoccele. The full-grown trophozoite is of relatively large size 

 and definite form, with a thick cuticle as a rule. In addition to 

 these characters, the reproduction and spore-formation, presently 

 to be described, are quite distinctive in type, the most diagnostic 

 feature being that each spore is the product of a single zygote. 



The Gregarines are an extremely abundant order of the Sporozoa, 

 highly differentiated in structure, and comprising a great number 

 of species classified into genera and families. They occur most 

 commonly as parasites of the digestive tract or body-cavity of 

 insects, but also as parasites of other classes, such as Echinoderms 

 and Annelids ; in Molluscs they are comparatively rare, and, though 

 they occur commonly in Prochordata (Ascidians), they are not 

 known from any class of Vertebrata in the strict sense of the word. 



In the early phases of development, during which the tropho- 

 zoite is a cell-parasite, it may be entirely enclosed in the cell, or 

 only attached to it by one extremity of the more or less elongated 

 body. In the latter case the sporozoite may have the anterior 

 end of the body modified into a definite rostrum, by which it attaches 

 itself to the host-cell, and from which is developed a definite organ 

 of attachment, termed an epimerite (Fig. 142, ep.), often of com- 

 plicated structure, and provided with hooks and other appendages. 

 When the cytozoic phase is past and the host-cell is exhausted, the 

 parasite drops off, shedding its epimerite as a rule. In the earlier 

 phase, in which an epimerite is present, the parasite was termed 

 by Aime Schneider a cephalont (" cephalin "), and in the later 

 phase a sporont (" sporadin "), the original use of this term, now 

 applied in a wider sense to denote in this and other orders of Sporozoa 

 those individuals about to proceed to spore-formation. The body 

 of the Gregarine-sporont always contains a single nucleus, but may 

 be divided into partitions or septa formed as ingrowths of the 

 ectoplasm, and is then said to be " septate " or " polycystid." 



* The subclass Rhabdogenise, as instituted by Delage and Herouard, included 

 the Sarcosporidia, which, however, are almost certainly true Amoebogeniae. 



