464 



ANNELIDA. 



found on the lateral portions of the segments, and are called the 

 lateral line organs. 



The pigmented bodies, called eyes, and occurring in pairs on some 

 of the segments of PolyopTithalinus, are probably photogenic organs. 



Eyes.* Paired eyes are often present on the prestomium, especially 

 in the free-living forms; but they are also found in other places, 

 e.g., the anal segment (Myxicola, Fdbricia), the gills of some 

 Sabellids (Branchiomma, Dasychone). The cephalic eyes attain a 

 large size and complex development in the pelagic Alciopidae^ 



a 



FIG. 369. Brain and anterior portion of the ganglionic chain of a, Serpula ; b, Nereis (after 

 Quatrefages). eyes; G cerebral ganglion (brain); c oesophageal (or pharyngeal) com- 

 missures ; Ug suboesophageal ganglion ; e, e' nerves to the tentacular cirri and peristomium. 



where they have a large lens and a complex retina (Fig. 389). In 

 most forms the prestomial eyes are simple cups of the epidermal 

 layer, which may be open or filled by a substance continuous with 

 the outer cuticle and constituting a lens, as in the eye of some 

 Gastropoda (vide Fig. 290, B). 



The vascular system is constructed on the usual type ; there are 

 dorsal and ventral contractile vessels giving off vessels to the skin 

 and internal organs. In some forms there is a continuous blood 

 sinus round the intestine instead of the usual capillary plexus 



* E. Andrews, " Eyes of Polychaetous Annelids," Journ. Morph. 7, 1892, 

 p. 169. 



t Greef. " Ub. d. Auge d, Alciopiden," etc., Marburg, 1876. 



