INFUSORIA. 833 



The protoplasm is either similar throughout, or it is divisible into an 

 exoplasm (cortex) and endoplasm (medulla). In the former case it is 

 variably granular, its movements slow as in Oxytrichidae and Euplotidae, 

 and it is sometimes filled with non-contractile vacuoles 1 . When there is 

 an exoplasm the transition from it to the more fluid endoplasm is gradual 

 or abrupt. The granular character of the endoplasm depends much on 

 the state of nutrition : the granules are albuminous, fatty, composed of 

 glycogen, sometimes doubly refractile, occasionally distinctly crystalline and 

 apparently formed of a urate. A mouth or cytostome with an oral tube, 

 and an anal spot or cytopyge, are absent only in the endoparasitic 

 Opalinidae. The former may be terminal and anterior, even mounted on 

 a protrusible eminence, e. g. Actinobolus, usually on one surface, hence the 

 ventral, and frequently in connection with a peristome (supra). The cuticle 

 lines the oral tube, which may have some extent. In the fixed Peritricha, 

 e.g. Vorticella, there is a vestibule common to the oral tube, the anal 

 spot and the duct of the contractile vacuole. The oral tube may be armed 

 with cuticular stylets, protrusible (Prorodon, Chilodoii), or even discharged 

 upon the prey (the Peritrichan Didinium) : or with more complex apparatus 

 (Dysteria armatd]. In the fixed Peritricha a long vibratory seta, the 

 optical expression according to some authorities of an undulatory mem- 

 brane, projects from the vestibule. Food is conveyed into the oral tube 

 by suction in carnivorous species, by the adoral band of cilia, which is 

 prolonged into it in other instances, and it is then lodged in a food- 

 vacuole. The anal spot is as a rule only visible at the moment of dis- 

 charge, but in the parasitic Holotrichan Nyctotherus, and perhaps some 

 others, there is an anal tube with cuticular lining. The spot is terminal 

 or subterminal, lateral in Stentor and Folliculina^ in the vestibule of Vorti- 

 cella and its congeners 2 . 



The protoplasm is generally colourless. It may be tinted by the food, 

 e.g. Oscillatorians in Nassula, by a diffused pigment, such as the blue 

 Stentorin of Stentor coeruleus, which yields a special absorption spectrum, 

 the sea-green of St. multifortnis, the blackish of St. niger, and dark-green 

 of Folliculina, the yellow or brown of many species ; or by pigment 

 particles, such as the crimson globules of Plolosticha flavo-rubra, the 

 crimson granules of St. igneus, mixed with chlorophyl bodies which occur 

 in that Heterotrichan, in St. polymorphus, Ophrydium versatile, and some 

 others. Certain of the green-coloured forms, e. g. the last named, occur 

 also colourless, and it has been supposed that the chlorophyl bodies are 

 really symbiotic algae 3 . The exoplasm when differentiated is always 



1 Trachelius ovum {Holotrichd) ; Bursaria truncatella, Stentor coeruleus {Heterotricha) ; 

 Loxodes rostrum, Condylostoma patens {Hypotricha}. 



2 On the supposed intestinal tube of Didinium, see Maupas, A. Z. Expt. (2) i. p. 597. 



3 See pp. 242-5. Add to the authorities there quoted, Engelmann, Pfliiger's Archiv fur 



3H 



