154 COMPARATIVE ANATOMY. 



We can bo more certain about function when tlio pigment-spot is 

 merely a covering for the special end-organs of the sensitve nerve. 

 These end-organs have the form of specially modified cells, which 

 traverse the pigment either singly or in groups ; judging from what 

 obtains in cases where the optic organs are more exactly known, we 

 may confidently say that these structures are in direct connection 

 with nerves. They are the so-called crystalline rods, or crys- 

 talline cones. 



Eyes of this kind are common enough in the Turbellaria, among 

 the Platyhelminthes (species of Mesostomum and Vortex) ; they 

 are as a rule found in pairs on the upper surface of the head. 

 Many marine Planaria have a larger number of regularly-ar- 

 ranged, well-defined, pigment-spots on this pai-t, some of which 

 surround a crystalline body. These eyes frequently appear in the 

 early stages of the embryo as pigment - spots ; this is also their 

 condition in many Trematode larvas, although, in many, distinct 

 crystalline bodies may also be made out (Amphistoma subclavatum, 

 Monostomum mutabile). In the entoparasitic forms of this division 

 the visual organs disappear, while they are persistent in many of 

 the ectoparasitic forms (Dactylogyrus). They are also persistent 

 in Polystomum. They are absent in all stages of the Ccstoda, 

 unless, indeed, we are to regard the red pigment-spots, which in 

 some lie behind the suckers, as rudiments of such organs. 



In the Nomertina, where eye-spots are frequently present, true 

 eyes have been observed in but few cases (Polia coronata, Nemertes 

 antonina). Eye-spots and true eyes of simple form are found on 

 their oesophageal ring, in the free living Nematodes (Enoplus), while 

 they are absent in nearly all the parasitic forms ; here, therefore, 

 degeneration of sensory organs goes hand in hand with parasitic 

 habit. 



The visual organs in the Rotatoria are placed immediately on the 

 cerebrum. There is one ciystalline rod to each of two connected 

 pigment-spots; or there is only a single visual organ with one 

 ' crystalline rod. Others have a pigment-spot and nothing else. 



The complex pair of eyes in Sagitta is distinguished by a large 

 number of radially-arranged crystalline cones, and with these are 

 found characters which remind us of what obtains in the Annulata. 



§ 125. 



Among the Annulata the optic organs of the Hirudinea occupy 

 the lowest position. The eyes, which are present in many, lie, as in 

 the Platyhelminthes, on the surface of the cephalic portion of the 

 body, and are, as in them, generally arranged symmetrically and in 

 large numbers. In their structure they agree in so remarkable a 

 manner with the cup-shaped structures mentioned in speaking of the 

 tactile organs, that in them a condition appears to exist, in which 

 a specific sensory organ is evolved from the indifferent 

 organs of sensation, which are found in the integument. 



