Introduction to Animal 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 nerve, 

 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. 



tion of body of Worm (pcristomium) ; n, cuticle; />, dormis ; r,/, dorsal and 

 ventral muscular platc-s ; ,/, opening of the scpmental orj^an ; k, coeloma; //, dorsal 

 ~fl; ,c, intestine; M, sexual organ ; c, nerve ganglion. B, section 

 of Amphioxus. C, section of vertebrate embryo. 



pigment spot, and diverging crystal cones on it. 

 -t in the tail segment in Fabricia, Amphi- 

 corina, and Amphiglena. Polyophthalmus has three 

 prostomial <-y<>s resting on the ganglion, the middle 

 with tlin-c, the lateral with two, crystal cones, i 

 succc'edm^- rni-tairn-n- has also a pair of eyes sunk in 

 its skin, and rerrivin^' a n<-rv<- from the ^'an^lion of 



-nt. Myxicola has also metameric eyes. In 



ttd AlciO] are lar^-e, with a retina, 



choroid, iris, and 1,-ns. Otocysts exist as simple trans- 



al rin^ of Arenicola 



