Rotifer a. 157 



a leech. The mouth leads into a muscular pharynx, furnished 

 "with several small jaws, which are continually being struck one 

 against the other. The digestive tract is usually short and 

 simple, and the anus is generally dorsal, at the base of the 

 foot, though in some forms it is wanting. The nervous system 

 is of the flat-worm type, cerebral ganglia anteriorly with nerves 

 springing from them ; one or two eyes are often present at the 

 front end. The excretory organs closely resemble those 

 of the Platyhelminths ; there is a pair of principal tubes with 

 smaller branches, whose club-shaped terminal swellings are like 

 those of the Plat-worms. The principal ducts open, after having 

 united to form a contractile vesicle, into the hinder part of the 

 gut or cloaca. There is no vascular system. The Rotif era 

 are of separate sexes. In a few forms only are the sexes alike, 

 usually they are extraordinarily different ; the males are smaller than 

 the females, they are entirely destitute of a mouth, and the digestive 

 tract is rudimentary, so that they can take in no nourishment ; 

 they live for only a few days ; they have no lorica, even when the 

 female has one, and the trochus is small; thej'' are less numerous than 

 the females, and in many genera were long unknown. The short 

 oviduct * opens, as a rule, into the cloaca ; the vas deferens dorsally 

 at the base of the foot. The Rotifera lay two different kinds of 

 eggs, viz., thin-shelled, unfertilised summer eggs, which 

 develop parthenogenetically ; and the thick-shelled fertilised 

 winter eggs, which do not hatch until long after they are laid. 

 The young ones do not undergo a metamorphosis. 



In a common fresh-water Rotifer, Hydatina senta, two kinds of females are 

 found. The one does not copulate and lays only summer eggs, wHch give rise 

 to females; the other is capable of copulating, and when this takes place, lays 

 winter eggs, or if this does not occur, summer eggs, which hatch into males. 

 Whether similar conditions ohtain for other foi-ms is stUl uncertain, although 

 probable. 



Much doubt exists as to the systematic position of the Rotifers ; they have 

 been sometimes placed with the Ai-thi-opoda, sometimes with the "Vermes." 

 From the facts which have lately come to light with regard to their stmcture, 

 it is probable that they must have been derived from the Platyhelminths {see 

 especially the excretory apparatus). 



The Rotifers are usually active and free-swimming, some, however, 

 are fixed. For the most part they live in fresh-water, a small number 

 in the sea. Some forms which occur in damp earth or on plants 

 (moss) can withstand complete desiccation, but this is not the case 

 with those living in water. A few are parasitic. 



* The (usually impaired) ovary is connected with a yolk-gland which produces a 

 mass of yolk, to be absorbed by the owun. 



