PROTOZOA AND OTHER ANIMALS 933 



is prolonged into two limbs hollowed into cups at the ends, the homo- 

 logues of halves of the scopula (Fig. 197A). The two cups are sealed 

 firmly together, so that the limbs form a closed ring by which the 

 ciliate is suspended (Fig. 197B) from the framework of the gills (Chat- 

 ton and Lwoff, 1923b, 1929). 



In Protozoa that live in association with other animals as hosts, the 

 developmental cycle must be adjusted to the requirements of the habitat. 

 This is so arranged as to insure transmission of the organism from one 

 host to another; production of a sufficient number of infective forms so 

 that the likelihood of some reaching a place where development can 

 continue is not too small; protection of the organism, if necessary, in the 

 period when it is out of its host; and often correlation with the life cycle 

 and habits of the host, so that escape from one host and infection of an- 

 other can take place. The situation is most complex in heteroxenous 

 forms, in which the life cycle is shared between two different species of 

 animals, and is correlated with the bionomics of each of them. Cyclic 

 adaptation has perhaps achieved its most perfect development when 

 there is a regular and direct transmission to the next generation through 

 infection of the eggs or embryos. There are not many instances in Pro- 

 tozoa of this last method, which is so perfectly exemplified among the 

 cyclic endosymbionts of insects (Buchner, 1930; W. Schwartz, 1935). 

 Nosema bombycis invades the eggs of silkworms, but this may be in- 

 cidental and is not the only method of transmission. Other instances 

 occur among heteroxenous Sporozoa in the invertebrate host in Babesia 

 and Karyolysis (see also Lavier, 1925). 



The developmental cycle and methods of transmission have been con- 

 sidered widely in textbooks and in a general way by many authors, in- 

 cluding Hegner (1924) and Grasse (1935). The situation in certain 

 epibiotic Protozoa is also of considerable interest; and in that connection 

 accounts are given below of the holotrichs Conidiophrys pilhuctor and 

 apostomatous ciliates. 



THIGMOTRICHA 



Chatton and Lwoff (1922c) proposed the name Thigmotricha for a 

 group (suborder) of holotrichs including the families Ancistridae, Hy- 

 pocomidae, and Sphenophryidae. Most of these ciliates are inquilines, 

 commensals, or parasites on the gills or palps of molluscs, though some 



