42 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 
@ on the head of the common freshwater worm 
; Nais. Each of these is simply an enlarged 
epidermal cell, along one side of which are 
several much smaller cells containing pig- 
ment (fig. 1054). We 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 
Eyegpernta Fechwater Avena refracting structures. Those of the Leech 
ened og oe’ 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. 1055.—Direction-Eyes of a Leech (Hzrudo, a), a Limpet (Patella, 8), 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 (eg.); it is enclosed in a pigmented 
sheath (Zg.), and consists of an external layer of large refracting cells (7.c.), surrounding a core of slender sense-cells 
(s.¢.), which are continuous with nerve-cells (z.c.), and these again with nerve-fibres (v.). B is an open cup, lined 
by a thickened retina (ve¢.) with clear refracting part externally, and dark pigment between its cells; #v., nerve. In 
c there are three lenses imbedded in pigment (Zg.), external to which are retinal cells (ve¢.c.), that contain refracting 
rodlets (vd.) 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 
