GENERAL STRUCTURE OF ROTIFERA. 



471 



themselves to any solid object ; and this is their ordinary position, 

 when keeping their '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, on the other hand, 



Fig. 244. 



remain attached by the pos- 

 terior extremity to the spot 

 on which they have at first 

 fixed themselves ; and their 

 cilia are consequently em- 

 ployed for no other purpose 

 than that of creating cur- 

 rents in the surrounding 

 w r ater. 



356. In considering the 

 internal structure of Roti- 

 fera, we shall take as its 

 type the arrangement which 

 it presents in the Rotifer 

 vulgaris (Fig. 244) ; and 

 specify the principal varia- 

 tions exhibited elsewhere. 

 The body of this animal, 

 when fully extended, pos- 

 sesses greater length in pro- 

 portion to its diameter than 

 that of most others of the 

 Class ; and the tail is com- 

 posed of three joints or 

 segments, which are capable 

 of being drawn -up, one 

 within another, like the 

 sliding tubes of a tele- 

 scope, each having a pair 

 of prongs or points at its 

 extremity. Within the ex- 

 ternal integument of the 

 body are seen a set of longi- 

 tudinal muscular bands (A), 

 which serve to draw the two 

 extremities towards each 

 other ; and these are crossed 

 by a set of transverse an- 

 nular bands, which also 

 are probably muscular, and 

 serve to diminish the dia- 

 meter of the body, and thus to increase its length. 



n 



Rotifer vulgaris, as seen at a with the 

 wheels drawn-in, and at b with the wheels 

 expanded: — a, mouth; b, eye-spots; c, 

 wheels; d, calcar [ antenna?); e, jaw and 

 teeth; /, alimentary canal; g, glandu- 

 lar (?) mass enclosing it ; h, longitudinal 

 muscles ; i, i, tubes of water-vascular 

 system ; k, young anima! ; I, cloaca. 



Between the 



