470 



GENERAL STRUCTURE OF ROTIFERA. 



general structure to be easily recognized. They have usually an 

 elongated form, similar on the two sides ; but this rarely exhibits 

 any traces of segmental division. The body is covered with a 

 double envelope, both layers of which are extremely thin and 

 flexible in some species, whilst in others the outer one seems to 

 possess a horny consistence. In the former case the whole integu- 

 ment is drawn together in a wrinkled manner when the body is 

 shortened ; in some of the latter the sheath has the form of a 

 polype-cell, and the body lies loosely in it, the inner layer of the 



integument being separated 

 Fig. 243. from the outer by a consider- 



able space (Fig. 246) ; whilst 

 in others the envelope or lorica 

 is tightly fitted to the body, 

 and strongly resembles the 

 horny casing of an Insect or the 

 shell of a Crab, except that it 

 is not jointed, and does not 

 extend over the head and tail, 

 which can be projected from 

 the openings at its extremities, 

 or completely drawn within it 

 for protection (Fig. 247). In 

 those Rotifera in which the 

 flexibility of the body is not 

 interfered with by the conso- 

 lidation of the external integu- 

 ment, we usually find it capable 

 of great variation in shape, the 

 elongated form being occa- 

 sionally exchanged for an 

 almost globular one, as is 

 seen especially when the animals 

 are suffering from deficiency 

 of water ; whilst by alternating 

 movements of contraction and 

 extension, they can make their 

 way over solid surfaces, after 

 the manner of a Worm or a 

 Leech, with considerable ac- 

 Brachionus pala. tivity, — some even of the 



loricated species being ren- 

 dered capable of this kind of progression by the contractility of 

 the head and tail. All these, too, can swim readily through the 

 water by the action of their cilia ; and there are some species which 

 are limited to the latter mode of progression. The greater number 

 have an organ of attachment at the posterior extremity of the 

 body, which is usually prolonged into a tail, by which they can affix 



