36 



INVERTEBRATE MORPHOLOGY. 



Fig. 18. — Acineta 

 grandis (after 

 Saville-Kknt). 



forms, e.g. Acineta (Fig. 18), are attached to foreign bodies by 

 a stalk. They do not possess any mouth, but a number of 

 simple or branched stiff processes project 

 from the body which serve for the prehension 

 and digestion of the organisms upon which 

 they feed. A contractile vacuole, nucleus, and 

 micronucleus are always present, the nucleus 

 having sometimes a very complicated shape. 

 It seems pretty clear that they have been de- 

 rived from the Ciliata, since in their young 

 stages they are .free-swimming ciliated struc- 

 tures ; the tentacular processes have been 

 compared to the pseudopodia of the Khizo- 

 pods, but good reasons for such an homology 

 do not exist, and it is more probable that they 

 are structures peculiar to the group. 



The Beproduction of the Infusoria. — ^In the 

 Infusoria the reproductive processes reach a 

 much higher grade of complication than occurs 

 in other Protozoa, though the simple pro- 

 cesses of fission and spore-formation likewise occur. The 

 former occurs in the majority of forms, and may be the only 

 mode of reproduction occurring throughout a number of gen- 

 erations. Long-continued fission seems, however, to lead in 

 many cases to structural and physiological derangements, 

 unless the process of conjugation be interposed. 



Encystment is also of frequent occurrence and may occur 

 under various conditions. In Colpoda, in which the process 

 has been most thoroughly studied, encystment may or maj' 

 not be followed by reproduction. In the latter case the cyst, 

 a resting cyst, is perfectly closed, and the walls are thick and 

 resistent so as to withstand unfavorable conditions, such as 

 insufficient aeration or dryness. When reproduction is asso- 

 ciated with encystment it may be either fission or spore-forma- 

 tion. The division cyst is thin-walled and is not completely 

 closed, and within it the animal undergoes division into two 

 or four parts. In spore-formation a thin cyst is first formed, 

 within which the animal slowly rotates, at the same time 

 gradually growing smaller by the expulsion of fluid. Finally 



