150 COMPARATIVE ANATOMY CHAP. 



The eyes are simple pigment spots in many Rhabdoccdidce ; in others 

 a refractive body is added. The eyes of the Triclada, Polyclada, and 

 ectoparasitic Trematoda are somewhat more complicated. They consist 

 of a pigment cup (often unicellular), at whose aperture lie one or more 

 nerve or retinal cells as perceiving elements. In the cavity of the eye- 

 cup lie homogeneous non-nucleated rods or clubs, apparently processes 

 of the retinal cells. A fine optic nerve is connected with the group 

 of retinal cells. In the Polyclada, where there are numerous eyes, the 

 sensory nerves from the brain branch, sending a single branch to each eye. 



B. Auditory Organs. 



These are not widely spread among the Platodes. They are only 

 found in the Rhabdocoelidce, and there almost exclusively in the division 

 of the Acoela, and among the Alloiocoda in the family of the Monotidce. 

 They are always found singly, and lie on the brain, and consist of a 

 small spherical vesicle filled with fluid ; an otolith or auditory stone 

 (composed of carbonate of lime and an organic substratum) is also 

 enclosed in this vesicle. 



C. Organs of Touch. 



These are universal in the Turbellaria. In the first place the skin 

 is everywhere very sensitive. This sensitiveness is caused by the 

 presence of delicate tactile hairs or tufts of immobile tactile hairs, 

 which are found in great numbers principally at the exposed parts of 

 the body, especially at the anterior edge and on the tentacles. The 

 tentacles may be considered in a special manner to be organs of touch ; 

 they are present in very many Polydada, but less frequently in Triclada 

 and Rhabdoccelidce. In the Polydadan family of the Planoceridce we find 

 on the dorsal surface, between the middle and the anterior end, two 

 lateral, mobile, stylet-shaped, solid tentacles (Fig. 103, t, p. 141), which 

 can occasionally be withdrawn into temporary depressions of the skin. 

 They may be directly compared with the tentacles of Ccelo- and Cteno- 

 plana, but are distinguished from these and from the contractile ten- 

 tacles of the Ctenophora (1) in that they have moved from the middle 

 of the back more or less far towards the front, and (2) in that they 

 have no lateral branches. In the Pseudoceridce and Euroleptidce there 

 are at the anterior edge tentacular folds of the leaf-shaped body, into 

 which branches of the intestine generally penetrate. In a few Triclada 

 also, and in Vorticeros among the fihabdocoelidce, feeler-like projections 

 or thickenings at the anterior margin of the body have been described. 



The so-called proboscis is a highly developed specific organ of touch 

 which distinguishes a Rlwbdocodan family, the Proboscidea. The anterior 

 end of the body of many Turbellaria is very retractile ; in Mesostoma 

 rostratum it can be telescopically withdrawn and protruded. The 

 permanent arrangement in the Proboscidea can be deduced from such a 



