26 ZOOLOGY 



of a spherical form, and grooved on one side like a peach. From 

 the bottom of this groove a very large transversely- striated 

 fiagellum takes its origin. Near the base of the flageUum is 

 the mouth, opening into a sort of pharynx ; a second smaller 

 flageUum has its origin in the latter. The protoplasm of the 

 globular body is very reticulate. No contractile vacuole has 

 been observed. At times NoctiLuca withdraws its flagella 

 and passes into a resting condition, but it does not form a cyst. 

 This animal is iateresting, as it is phosphorescent, and gives rise 

 to a large part of the phosphorescence of temperate seas. The 

 seat of the light is said to be the superficial protoplasm. 

 Eeproduction is by fission, but motile swarm-spores are also 

 formed ia large numbers. Conjugation has been observed. 



Class III. Ciliata. 



This class is characterised by the possession of ciHa as 

 locomotor organs, arranged either in a perioral ring, or forming 

 a more or less complete covering. A nucleus is always present ; 

 this may be single, and is then accompanied by a paranucleus, 

 or it may be distributed ia small fragments throughout the 

 body. One or more contractile vacuoles are present. The 

 shape of the body is very various. Some Ciliata are united 

 into colonies, and some form gelatinous tubes in which they 

 live ; the majority, however, are free-swimming. Conjugation is 

 common, but usually does not end in permanent fusion. Fission, 

 usually simple, but sometimes multiple, is the usual method of 

 reproduction. 



This class has been divided into four Orders, characterised 

 by the arrangement and nature of the ciha. 



Order I. Peritricha. — Cilia a/rranged in an anterior ring 

 or spiral, to which a posterior ring may he added. The rest of the 

 hody unciliated. 



Torquatella has its cilia fused to one another, and so a 

 vibratUe membrane is formed which surrounds the anterior end. 

 Trichodina is very common, crawling on the tentacles and body- 

 wall of Hydra ; it is pyramidal in shape, and has two circlets 

 of cilia. On its sucker-like base a curious horny toothed ring 

 is situated. 



