CHAETOPODA. 



599 



Polyophthahmis possesses in addition to cephalic eyes a number of lateral 

 eyes (twelve pairs in P. pictus\ one pair to a somite, situated 'ventrally 

 to the insertion of the oblique dorso- ventral muscles (supra] : Leptochone 

 aesthetica (Eriographidae among Serpulidae] has a pair on each somite ; 

 Fabricia (Sabellinae) a pair on the anal somite ; and many species of 

 Sabella (sometimes distinguished under the generic name Branchiomina) 

 and the genus Dasychone have them on the branchial filaments. 



Each eye of Nereis cultrifera consists of a hollow sphere filled with a 

 coagulable fluid. Its walls are composed of a single layer of cells : the 

 cells of the anterior or outer aspect are flattened and clear, those of 

 the posterior or inner aspect or retinal cells, to which the nerve is dis- 

 tributed, elongate, and of two kinds commingled, one pigmented, the other 

 not pigmented. The hypodermis cells immediately above the eye are 

 flattened ; around it, of great length, whilst the cuticle is unchanged. 

 There is a special optic ganglion for the anterior of the two pairs. The 

 structure of the two eyes of Alciope is similar but differs in the following 

 points. The inner ends of the retinal cells are capped by a rod (or two 

 semi-cylinders) and pigment (? in special cells) is massed at the spot where 

 the rods commence ; a spherical lens lies to the outer side of the cavity 

 of the sphere from which it is separated by a membrane. The cephalic 

 eyes <^i Polyophthahmis, which are imbedded in the cerebral ganglia, consist 

 of three clear bodies contained in a pigmented cup ; its lateral eyes possess 

 a hyaline lens, a layer of prismatic cells and nervous fibres inclosed in 

 a pigmented capsule, and both cuticle and hypodermis are thinner where 

 they pass over them. The eye of Hyalinoecia tubicola is said to consist 

 of a lens-like thickening of the cuticle with two layers of cells applied to it 

 (Pruvot). Otocysts are comparatively rare. A pair is found in the pro- 

 stomium of some Tubicola, e.g. Arenicola, Fabricia, some Sabellinae, and 

 young Terebellidae ; two or three pairs in the first setigerous somite of 

 Leptochone. Three lie near the eyes in the cerebral ganglia o>tPolyophthahnus\ 

 and the genus Aricia has a pair dorsally placed in a variable number of 

 the body-somites. The otocyst appears to consist of a capsule composed 

 of ciliated cells and containing one or more calcareous otoliths suspended 

 in a fluid. 



The mouth is sometimes terminal, more often ventral. It is a trans- 

 verse or longitudinal slit guarded by two lateral, or by upper and lower, 

 lips, and leads into a stomodaeum, which forms a buccal cavity, and in 

 most Polychaeta Errantia, some Tubicola, as well as in Oligochaeta, a 

 muscular pharynx. The latter is frequently more or less protrusible, Its 

 epithelium is sometimes ciliated and is generally covered by a cuticle : its 

 inner surface is sometimes papillate (e. g. in Syllidae\ and may be armed 

 with chitinoid teeth, either flattened toothed plates (Eunicidae) or pointed 

 cones (Aphroditidae, Nereidae]. Some Syllidae possess a pharyngeal tooth 



