INFUSORIA. 29 



may be respiratory or excretory. Kent considers it fully established that there is con- 

 stant and free intercommunication with the outer water ; that it is a mere pulsating 

 lacuna in the cortical layer of the ectoplasm, and that its leading function is excretory, 

 getting rid of the large quantity of fluid brought into the body by the ciliary or other 

 cuiTents incident to the capture and intussusception of the food. 



The contractile layer of the ectoplasm, i. e. that just beneath the cuticle, bears 

 permanent prolongations which are the organs of locomotion and prehension ; hence 

 they must pierce the cuticular layer to come in contact with the surrounding medium; 

 they are long, slender, and represent three well-marked forms, viz. : flagella, cilia, and 

 tentacula. The flagella are long whip-like protrusions of the body substance, often ex- 

 ceeding several times in length that of the body. They are never numerous — one, two, 

 or four are the most usual, although a larger number is not unknown ; their use, be- 

 sides propelling the creature through the water, is to assist in the capture of food 

 particles ; in sedentary species they produce currents in the water, directing them against 

 the body. In cases where more than one obtains, they are ordinarily in pairs, or sepa- 

 rate and situated at a distance from each other ; again, one flagellum, or one set, may 

 serve to anchor the animal, while another distantly situated may serve for food capture. 

 Cilia are short prolongations which resemble eye-lashes, hence the name ; it is by their 

 rhythmical and vigorous lashing of the water that the infusorian swims about so freely, 

 or, if it is fixed, by the same means water-currents are made to flow past the mouth, 

 and food is thus secured. The varioiis arrangements of these locomotory hairs will be 

 given under the description of the order Ciliata, named thus on account of their 

 characteristic natatory organs. Besides the vibratile cilia, there are other modifications 

 scarcely to be distinguished from them ; these are setae, or rigid hairs, used for support, 

 or for defence ; thick, straight setae, called stylets, usually situated beneath the body, and 

 uncini or curved hook-like hairs. The tentacula, in appearance and motion, at first 

 recall the pseudopodia of some of the Radiolaria, but more careful examination shows 

 that they are different. Each tentacle is tubular; a structureless external wall termi- 

 nating in a distal expansion, or sucker, encloses a core of gi-anular semi-fluid matter, 

 which is an extension of the endoplasm. These organs, situated promiscuously over 

 the animalcule's body, on well-defined areas, or on tubercles of the peristome border, 

 may be extended, retracted, or even bent at will. 



In the sinjplest of the Infusoria there is no constant aperture, or mouth, for the 

 reception of food, but, as in Amoeba, it is passed into the body substance indifferently 

 at any part of the periphery. It is plain that in silch cases a cuticle cannot be 

 present ; in others a certain definite portion of the siu-face receives the food. It is 

 safe to consider such forms more highly developed. Among those regarded as the 

 highest of the group, there is a well-defined oral aperture, often reinforced by ap- 

 pendicular appliances, and from which a passage, the oesophagus, leads into the 

 endoplasm. 



The multiplication of the Infusoria has been studied with much care. It will be 

 convenient to speak of the several methods : by binary division, by gemmation, by 

 spores, and by sexual reproduction. 



Examples of sub-division are frequently seen, even by the casual observer. The 

 process was accurately described in essentials by the earliest observers of these animals. 

 In a majority of instances it takes place across the body; after separation of the 

 nucleus into two parts, a constriction first appears at the middle, which increases in 

 depth until the two parts separate, forming two perfectly-formed, free-swimming indi- 



