ROTIFER A, THEIR GENERAL STRUCTURE. 



427 



FIG. 200. 



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 enve- 

 lope ; 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 integument is 

 drawn together in a wrinkled manner when the body is short- 

 ened ; 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 integu- 

 ment being separated from the outer by 

 a considerable space (Fig. 202) ; whilst 

 in others, the envelope or lorica is tightly 

 fitted to the body, and strongly resem- 

 bles 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 protec- 

 tion (Fig. 203). In those Rotifera in 

 which the flexibility of the body is not 

 interfered with by the consolidation of 

 the external integument, we usually 

 find it capable of great variation in 

 shape, the elongated form being occa- 

 sionally exchanged for an almost globu- 

 lar one, as is seen especially when the 

 animals are suffering from deficiency of 

 water ; whilst by alternative movements 

 of contraction and extension, they can 

 make their way over solid surfaces, after 

 the manner of a worm or a leech, with 

 considerably activity, some even of the 

 loricated species being rendered capable of this kind of progres- 

 sion, 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 attach- 

 ment at the posterior extremity of the body, which is usually 

 prolonged into a tail, by which they can affix themselves to any 

 solid object; and this is their ordinary position, when keepingtheir 

 wheels in action for a supply of food or of water. They have no 

 difficulty, however, in letting go their hold and moving through 

 the water in search of a new attachment, and may therefore be 

 considered as perfectly free; the polypoid species, however, 

 remain attached by the posterior extremity to the spot on which 

 they have at first fixed themselves, and their cilia are conse- 

 quently employed for no other purpose than that of creating 

 currents in the surrounding water. 



Brachion us pala. 



