304 ROTIFERA. 



vesicle, and are beset by short appendages called the vibratile tags 

 (Fig. 244). The latter appear to be simply flame-cells, which open 

 into the main trunks : whether they open into the perivisceral cavity 

 or not is disputed. 



The nervous system consists of a simple or bilobed cerebral 

 ganglion placed dorsal to the oesophagus, and giving off nerves 

 to peculiar cutaneous sense organs and to the muscles. In Calli- 

 dina and Discopus there is in addition a small ventral ganglion 

 connected with the dorsal ganglion rotund the oesophagus. Eyes are 

 often present, and lie upon the brain either as an x-shaped, unpaired 

 pigment body, or as paired pigment spots provided with refractile 

 spheres. There is often a small mass* of calcareous granules in 

 connection with the ganglion : its function is unknown. The 

 above-mentioned cutaneous sense-organs, which have the form of 

 prominences beset with hairs and setae, e.g. calcar, so-called 

 antennae, etc., are probably tactile. 



Generative organs. The sexes are separate, and are distinguished, 

 except in Seison, by a strongly marked dimorphism. The males 

 (Fig. 244, b) are much smaller than the females, and often have a 

 very different form ; they possess excretory tubes and ganglion, etc., 

 but are without either oesophagus or intestinal canal these organs 

 being reduced to a cord of cells : they leave the egg completely 

 developed. Their generative organs are reduced to a testicular sac, 

 the muscular duct of which opens at the hind end of the body, 

 sometimes on a papilliform protuberance. This is the penis, which 

 either introduces sperm into the female's cloaca in true copulation, 

 or perforates her body-wall and deposits the spermatozoa in the 

 body-cavity. 



The males of many species are unknown, and when they are 

 known they often appear only in the autumn, when the winter- 

 eggs are formed; but they are also found in summer. The generative 

 organs of the female consist of a roundish ovary, with which a 

 yolk-gland is associated, and of a short oviduct which usually opens 

 into the cloaca. The oviduct is sometimes absent (Rotifer), and 

 in the Pldlodinidae and Seisonidae the ovaries are double. Almost 

 all Rotifera are oviparous, and their eggs are distinguishable into 

 thin-shelled summer eggs, which develop immediately without being- 

 fertilized, and thick-shelled winter eggs, which last through the 

 winter and are probably fertilized. They carry both kinds of eggs 

 about on their body in their tube, but the summer eggs not un- 

 frequently develop in the oviduct. The summer eggs are of two 



