vin NERVOUS SYSTEM SENSE ORGANS 215 



medium, and carry off the carbonic acid formed in the tissues, and 

 so fulfil the act of respiration. This mechanism is physiologically 

 comparable with that of the contractile vacuole of fresh -water 

 Protozoa. In a few genera (Conochilus, Lacinularicc, Pterodina) 

 the kidneys open separately after a slight dilatation into the cloaca. 



Nervous System. The nervous centre of the Eotifera is the 

 brain (Fig. 112, C, &?), a ganglion lying dorsal to the pharynx ; 

 and when this is short it may be immediately below the surface 

 of the disc (Microcodori). In Bdelloida a second ganglion is present 

 below the pharynx, and is connected with the former by lateral 

 cords which contain ganglion cells. From the brain, nerves are 

 given off to the disc, to the muscles, and to the integument of 

 the body, as well as to the sense organs. The largest nerves are 

 two given off from the sides of the brain, each of which divides 

 into a lateral and a ventral trunk, which run nearly the whole 

 length of the animal. 



The brain of several Notommatidae has a curious appendage, 

 white by reflected light and very opaque ; it is a sac full of 

 chalky mineral matter, which dissolves readily in dilute acids. 



Sense Organs. The most widely diffused sense organs are 

 the antennae or feelers, which may serve for touch or smell, or 

 possibly both. Each antenna is a conical or tubular outgrowth 

 of the skin ; from its apex projects a fine pencil of sense hairs 

 borne on a protoplasmic cushion, which receives a nerve. Often 

 the antenna is elongated, and may then contain a muscle by 

 which it is retractile (lateral antennae of Melicertci) ; sometimes 

 it is reduced to a slight prominence bearing the setae (dorsal 

 antenna of this genus). There are usually three antennae a 

 median dorsal (Figs. 109, B, a, and 112, C, ani) and two lateral 

 (Figs. 106, 1 1 2, C, and 1 1 5 , A, al\ often approximated towards the 

 ventral surface, and sometimes all but fused on the middle line, 

 or completely united (Conochilus dossuarius, Copeus caudatus). 1 



Most Eotifers possess an organ of sight. This in its simplest 

 form is a refractive globule seated in a red pigmented cup through 

 which the nerve passes ; in other cases it lies directly on the 

 brain. Very frequently the eye is paired (Figs. 112, B, and 

 115, A); and these paired eyes may lie on the brain, and then 



1 Similarly Hudson and Zelinka botlv.jegard the dorsal antenna as formed by 

 the coalescence of two antennae. These retain thei/ distinctness in Asplanchna ; 

 in some Bdelloida the single antenna is supplied by a pair of nerves. 



