176 



ZOOLOOT. 



Class III. — Rotatoria {Rotifers). 



General Characters of Rotifers.— The Rotifers, or wheel- 

 animalcules, are abundant in standing water, in damp moss, 

 etc., and in the ocean, and are so transparent that their in- 

 ternal anatomy can be studied without dissection, while they 

 are so minute, being from one fortieth to three hundredths 

 of an inch in length (f to f mm.), that high powers of the 



microscope are needed in 

 studying them. They are 

 of special interest from 

 the fact that after being 

 dried for months to such 

 a degi-ee that little if any 

 moisture is left in the 

 body, they may be revived 

 and become active. Pro- 

 fessor Owen has observed 

 the revivification of a 

 Rotifer after having been 

 kept for four years in dry 

 sand. 



As an example of the 

 ordinary type of Rotifer 

 we may cite Squamella 

 magnifled 200 oUongci (Fig. 121), which 

 is allied to BracMonus. 

 The characteristic organ 

 of the wheel-animalcules 

 is the velum (lb) or pair 



Pig. Vi\.— Squamella dblcmqa 

 diameters. A view from below; shell or cara- 

 pace (5, -s^ 5^) ; 5, the aDterior transverse edge 

 of the carapace ; s', the anterior, and .s^, the 

 posterior corners of the carapace ; ."J^, the border 

 of the oval, flat area which occupies the lower 

 face of the carapace ; Ih, the cilia-bearing velam 

 of the head; t, the fork of the tail (I') ; m, the 

 mouth ; ,;', jaws ; ./•, muscles which move ,; ; st. 



ix ; CT, the co 

 the excretory syste: 



c^'^, ni*, the left excretory vessels ; eg, eg , 

 two largely developed young. — After Clark, 



tomacii ; a>, the' contractile vesicle, or heart of of ciliated wheel-like flaiJS 

 ystem ; ct', c??2^ the right, and i . ^ „ ,, , \ 



- on each side of the head, 

 which is comparable to 

 the velum of the larval mollusk. By means of the rotatory 

 movements of this velum the creature is whirled swiftly 

 around. The body is broad and flattened, with the walls 

 often dense, chitinous, sometimes shell-like, and variously 

 sculptured, or the animal may be long and worm-like, as in 

 Rotifer vulgaris (Fig. 122). The body is composed of several, 



