POLYCHAETA. 



463 



Ph 



the prestomium, and the eyes, when present, are often sessile upon 



it. The ventral cord is double, and the two halves are usually 



closely approximated, though in some Sabelliformia (Serpulidae) 



they are widely separated (Fig. 369), 



especially in front, and connected together 



by transverse commissures which pass 



between the ganglionic swellings. These 



latter are segmental, the first or sub- 



pharyngeal being often contained in the 



third segment, and the nerves are given 



off from them. In some forms, e.g. 



Arenicola, the ganglionic enlargements 



are very inconspicuous. A stomatogastric 



or visceral system is given off from the 



cerebral ganglion, or from the circum- 



pharyngeal commissures, or from both. 



Giant fibres are often present on the 



dorsal side of the ventral cords. The 



nerves to the palps arise from a special 



section of the cerebral ganglion ; the 



nerves to the tentacular cirri come off 



from the subpharyngeal ganglion, and 



sometimes from the circum-pharyngeal 



commissures as well. The subpharyngeal 



ganglion, which also supplies the first- 



parapodium, seems, as a rule, to consist 



of the ganglia of two or more segments 



fused. 



Sense organs. The tentacles, palps, 

 and tentacular cirri must be regarded as 

 tactile organs ; and the ciliated pits which 

 are often present on the prestomium and abut upon a special lobe of 

 the brain (Capitellidae, Oplieliidae, Arenicola, etc.) as olfactory. In 

 addition to these we sometimes find otocysts in the prestomium 

 (Arenicola and Polyophthalmus), or in some other segment of the 

 body (Fabrida, Myxicola, Terebella), or even segmentally repeated 

 in the dorsal regions of a certain number of segments (Aricia). 

 In some species of Arenicola they retain the opening to the 

 exterior.* 



In the Capitellidae groups of sensory cells bearing sense-hairs are 



* Ehlers, Z.f. w. Z., Sup. Bd. 53, 1892. 



FIG. 368. Alimentary canal of 

 Aphrodite aculeata (after M. 

 Edwards). Ph pharynx; D 

 intestine ; L intestinal (hepatic) 

 caeca. 



