TYPES OF PROTOZOA. 107 



frequent. The bell often divides into almost similar halves, 

 one of which may go free. Or one of the halves still attached 

 to the stalk may rapidly divide into eight small individuals, 

 which are then set adrift. Each of these, having a posterior 

 circlet of cilia, swims actively, and may conjugate with a 

 stalked individual of normal size. In this case, very different 

 from the conjugation of Paramcecium, the small individual 

 (like a spermatozoon) fuses wholly and intimately with the 

 other, which in its larger size and passivity may be likened 

 to an ovum. 



Gregarina — a type of those Gregarinida or Sporozoa, in 

 which the cell is divided into two regions by a partition. 



Various species of Gregarina occur in the intestine ot 

 lobster, cockroach, and other Arthropods, as cellular parasites 

 when young, free within the gut when adult. They absorb 

 diffusible food stuffs from their hosts. The maximum size 

 is about one-tenth of an inch. There is a firm cuticle, 

 and delicate fibrils sometimes extend across the cell- 

 substance, producing an appearance a little like that of 

 striated muscle. The unit is divided into a larger nucleated 

 posterior region, and a smaller anterior region, and at the 

 apex there is usually a small dead cap, which is lost after the 

 attainment of free intestinal life. Underneath the cuticle 

 lies a definite cortical stratum of the cell-substance, within 

 which the contents are more fluid ; the nucleus is very 

 distinct, but there are no vacuoles. We may associate the 

 absence of locomotor processes, " mouth," and contractile 

 vacuoles, as well as the thickness of the cuticle and the 

 general passivity, with the parasitic habit of the Gregarines. 



Let us begin with a young form parasitic in one of the 

 lining cells of the gut ; it grows and emerges till it hangs 

 attached to the cell by its cap only ; it becomes free and still 

 increases in size. Two individuals often attach themselves 

 end to end, but the meaning of this is obscure. Encystation 

 occurs involving a single unit or two together, and from the 

 division of the encysted cell spores are formed. The cyst is 

 sometimes (as in G.blattarum) complex, with " ducts" serving 

 for the exit of the spores, each of which is surrounded by 

 a firm case. Eventually the cyst bursts, the spore-cases 

 are liberated, and from within each of these the spore 



