ROTATORIA. 223 



Appendix (2} to Annelid Series. 



Class ROTATORIA. Rotifers. 



Rotifers are beautiful minute animals, abundant in fresh water, also found 

 in damp moss, and in the sea. They owe their name and the old-fashioned 

 title of wheel animalcules to the fact that the rapid movements of cilia 

 on their anterior end produce the appearance of a rotating wheel. The 

 food seems to consist of small organisms and particles caught in the 

 whirlpool made by the lashing cilia. The little animals are tenacious of 

 life, and can survive prolonged drought. If they are left dry for long, 

 however, they die, though the ova may survive and subsequently 

 develop. 



The body is usually microscopic, and is sometimes (e.g., in Melicerta 

 and Floscularid] sheltered within an external tube. There is no internal 

 segmentation, but there are sometimes external rings, and a ventral out- 

 growth or " foot " is sometimes segmented. The anterior end bears, on 

 a retractile ridge, the ciliated ring or "trochal apparatus." 

 / The nervous system is a single dorsal ganglion with a few nerves. An 

 unpaired eye and some tufts of sensory hairs are usually present. <\ 



The food canal extends along the body in a well-developed ccelomg, 

 and the fore gut contains a mill in which two complex hammers beat 

 upon an anvil. The canal ends posteriorly on the dorsal surface between 

 the body and the foot, and as the terminal portion also receives the 

 excretory canals and the oviduct, it is called a cloaca. 



There is no vascular system, but a nephridial tube of a primitive type 

 lies on each side of the body, and opens posteriorly into the cloaca. 



The sexes are separate ; the reproductive organs are simple. Except 

 in the marine parasite Seison and two other forms, the males are dwarfed 

 and degenerate, destitute even of a true food canal. In many cases at 

 least, sexual union (effected by a penis) seems to be ineffective, and there 

 is no doubt that many, if not most, Rotifers are parthenogenetic. The 

 females lay three different kinds of eggs, according to their conditions 

 and constitution either small ova, which become males, or thin shelled 

 "summer ova," or thick shelled " resting or winter ova," the two last 

 developing into females. Many species, however, are viviparous. We 

 include the Rotifers beside the Annelids proper, because it seems possible 

 to regard them as derived from ancestors somewhat like Annelid larvae. 



Rotifers living in fixed tubes or envelopes, Melicerta, Floscularia, 

 Stephanoceros. 



Free Rotifers, Notommata, Hydatina, Brachiomis. 



Parasitic on the marine Crustacean Nebalia, Seison. 



Pedalion occupies a unique position ; it has hints of appendages and 

 a peculiar jumping motion. 



Equally incertce sedis, but plausibly regarded as a specialised Trocho- 

 sphere, is the genus Dinophilus, with the nature of which advanced 

 students should make themselves acquainted. 



At this stage I may also mention that there are several sets of small 

 worm-like animals of whichjwe know very little. It is quite possible 



