6io Memoirs of the Indian Museum. [Vol. V, 



Nephthys oligobranchia, sp. nov. 

 (Plate XXIV, figs. 12A-12C.) 



This species is unique in the collection, insomuch as it is represented by 

 specimens from the Chilka Lake and also from another locality, viz. the Cochin 

 Backwater, near Ernakulam, in the south-west of the Madras Presidency. 



The superficial resemblance of this species to Nephthys polybranchia is so strong 

 that at first it was regarded as a mere variety of that species, but a closer examina- 

 tion revealed such striking differences as to indicate that the two forms are not 

 closely related. The description will be confined to these points of difference. A 

 mature male specimen was 17 mm. long, consisting of 51 setigerous segments. • An- 

 other had 47 setigerous segments. 



The head (fig. 12A) is rectangular, and the length slightly exceeds the width. 

 At the posterior dorsal angles are two small rounded papillae. The brain is rounded 

 behind, with a deep posterior indentation, and carries two small black eyes. The 

 front margin is almost straight. The head in fig. 12A is distorted and widened, 

 owing to the protrusion of the pharynx. The latter organ has in the distal region, 

 14 rows of papillae, 3-5 in a row. In the anterior dorsal line there is a conspicuously 

 long median papilla, but no corresponding ventral papilla. The mouth is surrounded 

 by 16 bifid papillae. 



In small specimens the ist pair of feet are directed forwards, alongside the head, 

 which they slightly surpass, but in adults they point obliquely outwards, and do not 

 reach the front of the head. 



This species differs only slightly from N. polybranchia as regards the shape and 

 arrangement of the setae. Camerated setae are as a rule confined to the anterior 

 13-15 pairs of feet, but in one specimen they persisted as far as the 18th feet. The 

 setae which replace them subsequently in the anterior rows, differ slightly from those 

 of N. polybranchia, the blade being narrower and more uniform in width. 



There is a marked difference between the two forms in the structure of the feet. 

 The first fully formed branchia is on the 6th foot, that on the 5th foot being very 

 small. They cease abruptly on the 20th-23rd foot, and are quite absent from the feet 

 of the middle and posterior regions of the body. Thus in a specimen having 47 seti- 

 gerous segments, branchiae are present on 17 pairs of feet only, the 6th to the 22nd, 

 and one specimen with 51 setigerous segments has 30 posterior abranchiate pairs of 

 feet. 



A typical branchiate foot, the 10th, is shown in front view in fig. 12B. The 

 posterior lamellae are very small and are surpassed by the spinal lobes, so that in 

 front view they are not visible. The anterior lamellae resemble those of N. poly- 

 branchia, and the ventral cirrus is equally small. The dorsal cirrus is very insigni- 

 ficant, being represented only by a small lobe on the upper side of the base of the 

 branchia. 



The vascular system of the foot differs from that of the preceding species. The 

 vessel which penetrates the branchia, after emerging, sometimes for a considerable 



