NERVOUS SYSTEM AND SENSE-ORGANS 



the sort are present in many Worms and Molluscs, and some of 



them are less complex than those of a Jelly- Fish or Star- Fish. 



Nothing, for example, could be much simpler than the eye-spots 



a on the head of the common freshwater worm 



v 



Nai's. Each of these is simply an enlarged 

 epidermal cell, along one side of which are 

 several much smaller cells containing pig- 

 ment (fig. 1054). W*e may take as examples 

 of greater complication the eyes of a Leech, 

 a Limpet, and an Arrow-Worm, the nature 

 of which is sufficiently indicated in fig. 1055. 

 They essentially consist of a group of visual 

 or retinal cells, associated with pigment and 

 refracting structures. Those of the Leech 

 are particularly interesting, because they 

 closely resemble in structure certain organs 



of touch which are present in the skin of the same animal, differ- 

 ing from these, however, in being larger, surrounded with pigment, 

 and limited to the front end of the body. It is, in fact, a case of 

 tactile organs which are acquiring a new function. The simple 



Fig. 1054. Section through an 

 Eye-Spot of a Freshwater Annelid 



Fig. 1055. Direction-Eyes of a Leech (Hirudo, A), a Limpet (Patella, B), and an Arrow- Worm (Sagitta, c), 

 in section, and enlarged to various scales 



In A the elongated eye is placed below a transparent patch of the epidermis (ep.}\ it is enclosed in a pigmented 

 sheath (pg.}, and consists of an external layer of large refracting cells (r.c.), surrounding a core of slender sense-cells. 

 (s.c.), which are continuous with nerve-cells (n.c.}, and these again with nerve-fibres (.). B is an open cup, lined 

 by a thickened retina (ret.) with clear refracting part externally, and dark pigment between its cells; nv., nerve. In 

 c there are three lenses imbedded in pigment (J>g.}, external to which are retinal cells (ret.c.), that contain refracting; 

 rodlets (rd.) in their inner ends. 



eye-cups, of which one is to be found at the base of each tentacle 

 in a Limpet, are interesting for quite a different reason. For 

 they are almost certainly to be regarded as degenerate structures, 

 which have been greatly simplified as a result of adaptation to the 

 mode of life characteristic of their possessor. The activity of a 

 Limpet is practically limited to feeding excursions in the vicinity 



