706 



RADIATA. 



transparency of their textures ; and that the order Rotifera ought to constitute a separate class, and 



to have a place in the Articulated series. 



Rotifera. 



The animals of this class have usually an elongated form, and are perfectly symmetrical on the two 

 sides. At the anterior extremity, we observe one or more rows of vibratile cilia; these are frequently 

 arranged (as in the common Wheel Animalcule, Fig. 21, b), in a circular manner, and when they are 

 in motion, an appearance of revolving wheels is produced, from which the class derives its appellation. 

 In many species we find a prolongation of the body in front, extending beyond the ciliary apparatus; 

 this, which sometimes bears one or more red spots that are supposed to be eyes, may be regarded as a 

 head (Fig. 21, a). The body is covered with a double envelope, both layers of which are extremely 

 thin and flexible in many species, whilst in others tbe outer one seems to possess a horny consistence, 

 and may even contain siliceous matter. In the Stephanifcros Eichornii, this is so far detached from 

 the body, as almost to form a sort of polype-cell, like that of the Bowerbankia ; and the cilia, instead 

 of being disposed in circular rows, are mounted on arms like those of a polype ; so that the whole 

 animal bears a strong resemblance to the Bryozoa, with which it forms the connecting link. 



_^ jl The body of the Rotifer vulgaris not being enclosed in any such 



dense envelope, is capable of considerable extension, and has much 

 of the Vermiform aspect; this is increased when a slight contraction 

 draws the external membrane into transverse wrinkles, that seem 

 to indicate the segments of the trunk. The posterior extremity 

 is prolonged into a tail, possessing three joints, which can be 

 drawn up within each other. Within the external integument 

 there are four longitudinal bands running from end to end ; these 

 are probably bundles of muscular fibre, by the contraction of 

 which the body may be shortened. On the under surface of the 

 head is a projecting orifice, which is believed to act as a syphon 

 for the introduction of water into the general cavity for the pur- 

 pose of respiration ; the ciliary movement being destined to bring 

 food to the mouth, which is situated between the wheels. The 

 wide cesophagus terminates in a sort of gizzard (d), provided with 

 regular teeth at its entrance. These teeth are two in number on 

 each side, and are fixed upon hard jaws, moved by powerful mus- 

 cles, so as to work between each other. All the food which is 

 swallowed is submitted to their action, before it enters the first 

 stomach, and when the cilia are in operation, these jaws are al- 

 ways in regular movement. From the first stomach or gizzard 

 there passes off, in the Rotifer, a long straight intestine {ff,) 

 which terminates without any dilatation except near its close at 

 g, just at the commencement of the tail ; this tube is surrounded 

 Fig. 21 Wheel Animalcules; a, with the by a glandular apparatus, h, which may be regarded as a liver. 



wheels expanded ; K, with the wheels folded up and T ., -p, ..«, -l n i .1 • 1 



draw., in; a, the head with the eye-spots; 4, the In many other .Rotifera, however, we find the gizzard opening 



wheels; c, water-siphon; rf, masticating apparatus; . , .. , . . 1 1 j ,, j. .. 



<■, salivary glands, //.intestinal canai ; g, its dilated into a larger cavity, which may be regarded as the true digestive 



termination ; h, glandular apparatus surrounding it ; . , -vr .V • .• e j.1. • a j." aX. -1 



i, young ones nearly complete; k, eggs; i, tail. stomach. Near the termination of the intestine is the oviduct, 

 ny which are extruded the eggs, k, which are formed in the large ovaria. Besides the longitudinal 

 muscular bands, transverse lines may be observed to cross the body at intervals, which are 

 believed by Ehrenberg to be blood-vessels, passing off from a trunk that runs along the back, like 

 the dorsal vessel of Insects. A distinct nervous system unquestionably exists in the Rotifera; though 

 it cannot be seen in the Rotifer vulgaris so clearly as in some others of the group. 



The movements of these animals are very active and varied. Sometimes they attach themselves by 

 the tail, and set their ciliary apparatus in motion for the purpose of obtaining food ; in which condition 

 they bear a strong resemblance to Bryozoa. But they also swim freely about through the water, the 

 ciliary apparatus being folded up (as in Fig. 21, B), and they then resemble aquatic "Worms. And 

 sometimes they crawl along a solid surface, after the manner of a Leech. Yet with all this complex 

 organization and these active powers, many Rotifera may be completely dried up, and preserved for an 



