FLATWORMS AND MESOZOA, ETC. 59 



enclosing, by its mobile fringed margin, prey as large as Lepto- 

 plana itself. The upper division of the chamber communicates 

 by a hole in the roof* (the true mouth, g.m.) with the cavity of 

 the main-gut or stomach {m.g.), which runs almost the length of 

 the body in the middle line forwards over the brain {up). Seven 

 pairs of lateral gut-branches convey the digested food to the 

 various organs, not directly, however, but only after the food 

 mixed with sea-water has been repeatedly driven by peristalsis, 

 first towards the blind end of the gut-branches and then back 

 towards the stomach. Respiration is largely effected by these 

 means. 



The brain, which is enclosed in a tough capsule {pr.\ is placed 

 in front of the pharynx, but some distance behind the anterior 

 margin of the body. It is of an oval shape, subdivided super- 

 ficially into right and left halves by a shallow depression, and is 

 provided in front with a pair of granular-looking appendages, com- 

 posed of ganglion-cells, from which numerous sensory nerves 

 arise, supplying the eyes and anterior region. Posteriorly, the 

 brain gives rise to a chiefly motor, nervous sheath {n.n.), which 

 invests the body just within the musculature. This sheath is 

 thickened along two ventral lines {In.) and two lateral lines {n.s), 

 but is very slightly developed on the dorsal surface. 



Leptoplana possesses eyes, stiff tactile, marginal cilia, and pos- 

 sibly a sense organ in the " marginal groove." The eyes, which 

 are easily seen as collections of black dots lying at the sides of 

 the brain, may be divided into two paired groups : — i, Cerebral 

 eyes (^.), and 2, tentacle eyes (e.t.), which indicate the position of 

 a pair of tentacles in allied forms. Each ocellus consists of a 

 capsule placed at right angles to the surface of the body in the 

 parenchyma, below the dorsal muscles, and with its convex face 

 outwards. It is a single cell, in which pigment granules have 

 accumulated. The light, however, can only reach the refractive 

 rods, which lie within it obliquely at their outer ends. These 

 rods are in connection with the retinal cells, and thus communi- 

 cate by the optic nerve with the brain. The cerebral eyes are 

 really paired, and are directed, some upwards, some sideways, 

 some downwards. 

 *Thc roof of the peripharyngeal chamber is hence known as the "cliaphragm." 



