28 ZOOLOGY 



as in Paramoecium, and fission is oblique. Eecent observation 

 on this and other forms shows that as long as the products of 

 artificial division contain part of the nucleus with its chromatin, 

 they are capable of regenerating the lost parts ; those portions 

 of the body which are without any portions of the nucleus die. 

 BalaTdidiwm is a genus which lives parasiticaUy in the 

 human colon, and with Nyetotherus, is found in the rectum 

 of Anura, etc. The latter is interesting, since it is provided 

 with a permanent anus with a cuticular lining. In most Cihata 

 the situation of the cell anus is constant, but there is nothing 

 to indicate its position, except when waste matter is being 

 expelled. 



Order III. Holotricha. — In this order the body is uni- 

 formly clothed with short cilia, arranged in regular rows. Some- 

 times those on the adoral surface are slightly longer than the 

 others. 



Paramoecium is one of the commonest genera of this 

 order. Close underneath its cuticle in the ectosarc, is a layer 

 of oval bodies, the Trichocysts; these, when the animal is 

 irritated, discharge threads, which have probably the same 

 functions as the stinging threads in the nematocysts of 

 Coelenterata. 



Maupas has recently described the conjugation of Para- 

 moecium aurelia, which takes place as follows. As soon as two 

 individuals come together, the paranuclei, of which there are 

 two (Fig. 21, 1), separate themselves from the nucleus, increase 

 in size (Fig. 22, A), become spindle-shaped, and ultimately divide 

 into two (2 and B). The two halves of each divide again (C), so 

 that a stage is found with eight similar portions of the original 

 paranucleus in each individual (3 and 4). Of these eight 

 corpuscles, seven are absorbed and disappear (5) ; the eighth 

 alone, and this is always that one which lies nearest to the 

 mouth, undergoes further change. This corpuscle increases 

 in size and divides into two (6 and D), thus giving rise to the 

 male and the female pronucleus. The former of these passes out 

 of each conjugating individual into the other, and there fuses 

 with the female pronucleus (7 and E). The conjugating animals 

 now separate. The " fertilised " paranucleus now divides into 



