CHAP. I ANATOMY OF POLYCLADIDA I 5 



ginal cilia, and possibly 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 : 

 (1) cerebral eyes (Fig. 5, e), and (2) ten- 

 tacle eyes (et\ which indicate the position 

 of a pair of tentacles in allied forms (Fig. 

 8, A, t and .B). 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 con- 

 vex face outwards. It is a single cell 

 in which pigment granules have accumu- 

 lated. The light, however, can only reach 

 the refractive rods, which lie within it, T i^.-I>mgnm of an eye 

 obliquely at their outer ends. These rods of Leptoplana from the 



.,, , . , ,, , tentacle group, x 600. 



are in connexion with the retinal cells, and (After Laug.) 



thus communicate by the optic nerve with 



the brain. The cerebral eyes are really paired, and are directed 



some upwards, some sideways, some downwards. 



The " marginal groove " is a shallow depression of the 

 epidermis (Fig. 5, gr) lined by cilia, and containing the ducts 

 of very numerous gland-cells. It runs almost parallel to the 

 anterior margin of the body, a short distance from it, but we 

 have no observations on its functions. 



Reproductive Organs. Leptoplana is hermaphrodite, and, 

 as in most hermaphrodites, the reproductive organs are com- 

 plicated. The male organs are the first to ripen, but this does 

 not appear to prevent an overlapping of the periods of maturity 

 of the male and female products, so that when the eggs are 

 being laid, the male organs are, apparently, still in a functional 

 state. The principal parts are seen in Fig. 5. The very 

 numerous testes (te) are placed ventrally, and are connected with 

 fine vasa efferentia (ye), which form a delicate network opening at 

 various points into the two vasa deferentia (yd}. These tubes, 

 especially when distended with spermatozoa, may easily be seen 

 (Fig. -2, vd) converging at the base of the penis, and connected 

 posteriorly by a loop that runs behind the female genital pore 

 (Fig. 5). The penis (pe) is pyriform and muscular, and is 

 divided into two chambers, a large upper one for the sperma- 

 tozoa, and a smaller lower one for the secretion of a special 



