248 MANUAL OF ZOOLOGY. 



frog" (Bastian). This extraordinary history is rendered still more remark- 

 able, if it should be proved that the young of the parasitic forms of this 

 Ascaris are produced by a process of parthenogenesis; and this seems to 

 be highly probable, since none of the individuals which are found as para- 

 sites are males, but are universally females. 



CHAPTER XXVI. 



ROTIFER A, 



SUB-CLASS ROTIFERA (Rotatoria). The Rotifera, or " Wheel- 

 animalcules," constitute a very natural group, the exact posi- 

 tion of which has been a good deal disputed, and is still 

 doubtful. They are looked upon here as a distinct division 

 of the Scohcida ; but they are very frequently placed with the 

 Annelida amongst the next division 6f the Annulosa (Anar- 

 thropoda). 



The Rotifera are minute animals^ never parasitic, inhabiting 

 water, and usually provided with an anterior ciliated disc, capable 

 of inversion and eversion. In the females there is a distinct mouth, 

 intestinal canal, and anus. A nervous system is also presmt, con- 

 sisting of ganglia, situated near the anterior extremity of the body, 

 and sending filaments backwards. A water-vascular system is 

 also present. The sexes are distinct. 



Most of the Rotifera are entirely invisible to the naked eye, 

 and they are all extremely minute, none of them attaining 

 a greater length than i-36th of an inch. Nevertheless, as 

 remarked by Mr Gosse, "so elegant are their outlines, so 

 brilliantly translucent their texture, so complex and yet so 

 patent their organisation, so curious their locomotive wheels, 

 so unique their apparatus for mastication, so graceful, so 

 vigorous, so fleet, and so marked with apparent intelligence, 

 their movements, so various their forms and types of struc- 

 ture," that they form one of the most interesting departments 

 of zoological and microscopical study. They are all aquatic 

 in their habits, and in the great majority of cases are free- 

 swimming animals, some, however, being permanently fixed, 

 as is the case with Stephanoceros, Melicerta (fig. 122, B), and 

 Floscularia. They are usually simple, but are occasionally 

 composite, forming colonies, as in Megalotrocha. As a, rule, 

 the male and female Rotifera differ greatly from one aiother, 

 the males being smaller than the females, destitute of any 



