Page:Encyclopædia Britannica, Ninth Edition, v. 21.djvu/17

Rh KOTIFERA a Polyzoan larva, while Entcroplea is probably a male Rotifer, and, like the other males, iu 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 Ch&topods' 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- dinadse), 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 yellowisk- greeii or brown particles, and outside these a connective tissue membrane ; muscular fibrillae 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 intestine 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 nephridia and oviducts may open into it, and the eggs lodge there on their way outwards ; they are thrown out, as are the faecal masses, by an eversion of the cloaca. Asplanchna, Notommata sieboldii, and cer- tain species of Ascomorpha, are said to be devoid of intestine or anus, excrementitious matters being ejected through the mouth (11). Nephridia. The coelom 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 coelom 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 Infusorian. Nervous 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. Where 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 protrtisible 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 Rotifera 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 Eotifers, until at the present time degenerated males are known to occur in all the families except that of the Philodinadse. The male Rotifers are provided with a single circlet of cilia (a peritrooh), 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 eversible penis. The males of Floscularia are shown in fig. 1. The male of Pcdalion 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 tetronerythrin. 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, Notommata'), 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, &c., 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 miilleri, B. hepta- tonus, Synch&ta 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 larvae," and formed the connecting link between the Nemertidse and the Nematoid worms. Ray Lankester proposed to associate them with the Ch&topoda and Arthropoda in a group Appendiculata, 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 coelom, 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 Ch&topoda. But the high development of the mastax, the specialized character of the lorica in many forms, the movable spines of Polyarthra, 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, nou-retractile truncate. Fam. 1.' FLOscuLARiAD.fi. Floscularia, Stephanoceros. Fam. 2. MELICERTAD.S. i Melicerta, Cephalosiphon, Megalo- trocha, Limnias, ^Ecistes, Lacinularia, Conochilus.