92 



ROTIFERA 



a Polyzoan larva, while Eutcropka is probably a male Rotifer, and, 

 like the other males, in a reduced condition. There is no reason for 

 considering this mastax as the homologue of either the gastric mill of 

 Crustaceans on the one hand or the teeth in the Chajtopods' pharynx 

 on the other ; it is merely homoplastic with these structures, but has 

 attained a specialized degree of development. Both the pharynx 

 and the cesophagus which follows it are lined with chitin. The 

 oesophagus varies in length and in some genera is absent (Philo- 

 elinadae), the stomach following immediately upon the pharynx. 

 The stomach is generally large ; its wall consists of a layer of very 

 large ciliated cells, which often contain fat globules and yellowish- 

 green or brown particles, and outside these a connective tissue 

 membrane ; muscular fibrillre have also been described. Very 

 constantly a pair of glands open into the stomach, and probably 

 represent the hepato-pancreatic glands of other Invertebrates. 



Following upon the stomach there is a longer or shorter intestine, 

 which ends in the cloaca. The intestine is lined by ciliated cells. 

 In forms living in an urceolus the iutestine turns round and runs 

 forward, the cloaca being placed so as to debouch over the margin 

 of the urceolus. The cloaca is often very large ; the uephridia and 

 oviducts may open into it, and the eggs lodge there on their way 

 outwards ; they are thrown out, as are the fecal masses, by an 

 eversion of the cloaca. Asplanchna, Notominata sicboldii, and cer- 

 tain species of Ascomarplia are said to be devoid of intestine or 

 anus, excrementitious matters being ejected through the mouth (11). 



Nephridia. The ccelom contains a fluid in which very minute 

 corpuscles have been detected. There is no trace of a true vascular 

 system. The nephridia (fig. 2, B, n) present a very interesting 

 stage of development. They consist of a pair of tubules with an 

 iutracellular lumen running up the sides of the body, at times 

 merely sinuous, at others considerably convoluted. From these 

 are given off at irregular intervals short lateral branches, each of 

 which terminates in a flame-cell precisely similar in structure to 

 the flame-cells found in Planarians, Trematodes, and Cestodes ; 

 here as there the question whether they are open to the ccelom or 

 not must remain at present undecided. At the base these tubes 

 open either into a permanent bladder which communicates with the 

 cloaca or into a structure presenting apparently no advance in its 

 development upon the contractile vacuole of a ciliate Infusoriau. 



Nervmw System and Sense-Organs. Various structures have been 

 spoken of as nervous which are now acknowledged to have been 

 erroneously so described (18). There is a supra-cesophageal gang- 

 lion which often attains considerable dimensions, and presents a 

 lobed appearance (fig. 2, A and B, g). Connected with this are the 

 eye-spots, which are seldom absent. AVhere these are most highly 

 developed a lens-like structure is present, produced by a thicken- 

 ing of the cuticle. In the genus Rotifer and other forms these are 

 placed upon the protrusible portion of the head, and so appear to 

 have different positions at different moments. The number of eye- 

 spots varies from one to twelve or more. They are usually red, red- 

 dish-brown, violet, or black in colour. Other structures are found 

 which doubtless act as sense-organs. The calcar above-mentioned 

 generally bears at its extremity stiff hairs which have been demon- 

 strated to be in connexion with a nerve fibril. On the ventral sur- 

 face of the body just below the mouth a somewhat similar structure 

 is often developed the chin. There are besides at times special 

 organs, like the two lateral organs in Microcodon (fig. 1, D, s), which 

 no doubt in common with the calcar and chin have a tactile function. 



Reproductive Organs and Development. The Eotifera were 

 formerly considered to be hermaphrodite, but, while the ovary was 

 always clear and distinct, there was always some difficulty about 

 the testis, and various structures were put forward as representing 

 that organ. One by one, however, small organisms have been dis- 

 covered and described as the males of certain species of Rotifers, 

 until at the present time degenerated males are known to occur in 

 all the families except that of the Philodiiiadss. The male Rotifers 

 are provided with a single circlet of cilia (a peritroeh), a nerve 

 ganglion, eye-spots, muscles, and nephridial tubules all in a some- 

 what reduced condition, but there is usually no trace of mouth or 

 stomach, the main portion of the body being occupied by the testi- 

 cular sac. There is an aperture corresponding with the cloaca of the 

 female, where the testis opens into the base of an eversihle penis. 

 The males of Floscularia are shown in fig. 1. The male of Pedalion 

 mira possesses rudimentary appendages, The ovary is usually a 

 large gland lying beside the stomach connected with a short oviduct 

 which opens into the cloaca. The ova often present a reddish hue 

 (Philodina roseola, Brachionus rubens), due doubtless, like the red 

 colour of many Crustacean ova, to the presence of tetrouerythrin. 



Up to the present our embryological knowledge of the group is 

 very incomplete. Many Rotifers are known to lay winter and 

 summer eggs of different character. The winter eggs are provided 

 with a thick shell and probably require fertilization. Two or three 

 of them are often carried about attached to the parent (Brachionus, 

 Notominata), but they are usually laid and fall into the mud, there 

 to remain till the following spring. The summer eggs are of two 

 kinds, the so-called male and female ova, both of which are stated 

 to develop parthenogenetically. They may be carried about in 



large numbers in the cloaca or oviduct or attached to the body of 

 the parent. The female ova give rise to female and the male ova 

 to male individuals. Male individuals are only formed in the 

 autumn in time to fertilize the winter ova. 



Habitat and Mode of Life. The Rotifera are distri- 

 buted all over the earth's surface, inhabiting both fresh 

 and salt water. The greater number of species inhabit 

 fresh water, occurring in pools, ditches, and streams. A 

 few species will appear in countless numbers in infusions 

 of leaves, <tc., but their appearance is generally delayed 

 until the putrefaction is nearly over. Species of Rotifer 

 and Philodina appear in this way. A few marine forms 

 only have been described Brachionus mulleri, B. hepta- 

 tomis, Synchxta baltica, and others. 



A few forms are parasitic. Albertia lives in the intestine 

 of the earthworm ; a form has been described as occurring 

 in the body-cavity of Synapta ; a small form was also 

 observed to constantly occur in the velar and radial canals 

 of the freshwater jelly-fish, Limnocodium. Notommata 

 parasitica leads a parasitic existence within the hollow 

 spheres of Volvox globator, sufficient oxygen being given 

 off by the Volvox for its respiration. 



Many Rotifers exhibit an extraordinary power of resist- 

 ing drought. Various observers have dried certain species 

 upon the slide, kept them dry for a certain length of time, 

 and then watched them come to life very shortly after the 

 addition of a drop of water. The animal draws itself to- 

 gether, so that the cuticle completely protects all the softer 

 parts and prevents the animal itself from being thoroughly 

 dried. This process is not without parallel in higher 

 groups ; e.g., many land snails will draw themselves far into 

 the shell, and secrete a complete operculum, and can remain 

 in this condition for an almost indefinite amount of time. 

 The eggs are also able to withstand drying, and are pro- 

 bably blown about from place to place. The Rotifera can 

 bear great variations of temperature without injury. 



Since their removal from among the Protozoa various 

 attempts have been made to associate the Rotifera with 

 one or other large phylum of the animal kingdom. 

 Huxley, insisting upon the importance of the trochal disk, 

 put forward the view that they were " permanent Echino 

 derm larvas," and formed the connecting link between 

 the Nemertidx and the Nematoid worms. Ray Lankester 

 proposed to associate them with the Chxtopoda and 

 Arthropoda in a group Apptndicidata, the peculiarities in 

 the structure of Pedalion forming the chief reason for 

 such a classification. There is, however, no proof that we 

 thus express any genetic relationship. The well-developed 

 ccelom, absence of metameric segmentation, persistence of 

 the trochal disk in varying stages of development, and the 

 structure of the nephridia are all characters which point to 

 the Rotifera as very near representatives of the common 

 ancestors of at any rate the Mollusca, Arthropoda, and 

 Chxtopoda. But the high development of the mastax, 

 the specialized character of the lorica in many forms, the 

 movable spines of Polyarthm, the limbs of Pedalion, and 

 the lateral appendages of Asplanchna, the existence of a 

 diminutive male, the formation of two varieties of ova, all 

 point to a specialization in the direction of one or other of 

 the above mentioned groups. Such specialization is at 

 most a slight one, and does not justify the definite associa- 

 tion of the Rotifera in a single phylum with any of them. 



Classification. The following classification has been 

 recently put forward by Dr C. T. Hudson (19). 



CLASS ROTIFERA. 

 Order I. Rhizota. 



Fixed forms ; foot attached, transversely wrinkled, non-retractile 

 truncate. 



Fam. 1. FLOSCULARIADJE. Floscularia, Stcphanoceros. 

 Fain. 2. MELICERTADJE. , Meliccrta, Ccphalosiphon, Mcgalo- 

 troc/ta, Limnias, ^Ecistes, Lacinularia, Conochilus. 



