Introduction to A nimal Morphology. 



215 



of the anterior segments are sensitive, and may be 

 jointed. The ciliated grooves in Polygordius re- 

 semble those of Nemerteans. The eyes, usually on 

 the prostomium, consist of a pigment spot, a nerv^e, 

 and generally a lens, often 2-3 lenses on the one pig- 

 ment spot. They are rarely simple specks, usually 

 close to the ganglion. When there are four they are 

 rarely equal ; one pair is generally large, and the 

 other only pigment spots. Branchiomma has many 

 eyes on its tentacles, Sabella on its gills. Each has a 



Fig. 29. 



A, section of body ofWorm (peristomium) ; a, cuticle; b, dermis; c,y, dorsal and 

 ventral muscular plates ; d, opening of the segmental organ ; k, coeloma ; h, dorsal 

 vessel ; i, ventral vessel ; g, intestine ; n, sexual organ ; e, nerve ganglion. B, section 

 of Amphloxus. C, section of vertebrate embrj'O. 



pigment spot, and diverging crystal cones on it. 

 Eyes exist in the tail segment in Fabricia, Amphi- 

 corina, and Amphiglena. Polyophthalmus has three 

 prostomial eyes resting on the ganglion, the middle 

 with three, the lateral with two, crystal cones. Each 

 succeeding metamere has also a pair of eyes sunk in 

 its skin, and receiving a nerve from the ganglion of 

 the segment. Myxicola has also metameric eyes. In 

 Torrea and Alciope the eyes are large, with a retina, 

 choroid, iris, and lens. Otocysts exist as simple trans- 

 parent vesicles on the pharyngeal ring of Arenicola 



