154 MANUAL OP ZOOLOGY. 



The parasitic and free Nematoids are connected together by 

 an Ascaris (A.nigrovenosa^wlciich in succeeding generations is 

 alternately free and parasitic. This Ascaris has long been 

 known as inhabiting the lungs of the frog, but it has been 

 shown that ' the young of this animal become real, free ISTema- 

 toids ; for, after passing from the intestine of the frog into damp 

 earth or mud, they grow rapidly, and actually develop in the 

 course of a few days, whilst still in this external medium, into 

 sexually mature animals. Young, differing somewhat in exter- 

 nal characters from their parents, are soon produced by them, 

 and these attain merely a certain stage of development whilst 

 in the moist earth, arriving at sexual maturity only after they 

 have become parasites, and are ensconced in the lung of the 

 frog.' (Bastian.) This extraordinary history is rendered still 

 more remarkable, 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 parasites are males, 

 but are universally females. 



CHAPTER XXVII. 

 ROTIFERA. 



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

 animalcules,' constitute a very natural group, the exact position 

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

 They are looked upon here as a distinct division of the Scolecida, 

 following Huxley ; but they are very frequently placed with the 

 Annelida, amongst the lower division of the Annulosa (Anar- 

 thropoda^). 



The Rotifera are Annuloida of a minute size, never parasitic, 

 inhabiting water, o,nd 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 present, consisting of ganglia, situated near the anterior ex- 

 tremity of the body, and, sending filaments backwards. A water- 

 vascular system is also present. 



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

 and they are all extremely minute. Nevertheless, as remarked 

 by Mr. Gosse, ' so elegant are their outlines, so brillantly trans- 

 lucent their texture, so complex and yet so patent their orga- 

 nisation, so curious their locomotive wheels, so unique their 

 apparatus for mastication, so graceful, so vigorous, so fleet, 



