1921.] 



Fauna of the Chilka Lake : Polychaeta. 



577 



brane, the " dorsal fillet " /. Beneath the setae, and embedded in the upper part of the 

 median ligule, is the dorsal spine h. The dorsal fillet is attached by its edges to the 

 median ligule. Between the median and ventral ligules lie the " setae of the ventral 

 division," enclosed within the ventral fillet g. The latter projects outwards in two 

 lobes, a larger anterior and a smaller posterior. The "ventral spine" k terminates 

 in the anterior lobe of the fillet. 



Three types of setae are most commonly found in the Nereidae, and their position 

 in the foot is fairly constant, the arrangement in TV. pelagica being typical. Homo- 

 gomph setae with elongate finely- serrated terminal pieces (plate XXT, fig. 6k) occur in 



Text-fig. i. — Foot of Nereis pelagica, L,. 



A. 10th right foot, front view. B. 10th right foot, posterior view. 



C. 1 oth right foot, diagram. 

 The lettering is identical in the three figures. 



a = dorsal cirrus ; b = ventral cirrus ; c = dorsal ligule ; d = median ligule ; e = ventral ligule ; / = dorsal fillet ; g. 

 = ventral fillet ; h = dorsal spine ; h = ventral spins. 



• = Homogomph setae with elongate tips ; o = Heterogomph setae with elongate tips ; x = Heterogomph setae 

 with falcate tips. 



the dorsal division, and in the upper posterior part of the ventral division. Hetero- 

 gomph setae with elongate terminal pieces (plate XIX, fig. 2H) occur in the lower 

 posterior part of the ventral division. Heterogomph setae with short falcate termi- 

 nal pieces (plate XXI, fig. ys) occur in two groups on the anterior side of the ventral 

 division, one group above, and the other below, the spine. 



Variations from this typical structure take the form either of reduction of the 

 number of lobes on the one hand, or on the other of increased complexity due to 

 branching. The condition of the various genera may be passed in review. 



In Ly casus the foot is very simple, consisting only of dorsal and ventral cirri and 

 the setigerous divisions. This simplicity is probably not primitive, but derived, as 



