370 Memoirs of the Indian Museum. [Vol. V, 



(5) A thin layer of transverse muscle-fibres surrounding the inhalent and exhalent 



canals. This layer is much thicker in the situation of the transverse 

 partition than in the wall of the siphon. 



(6) A thin layer of longitudinal muscle fibres. 



(7) A thin layer of transverse muscle fibres. 



(8) A layer of columnar epithelial lining of the inhalent and exhalent canals. 



The structure of the wall of the siphon at the junction of the contiguous seg- 

 ments (text-fig. 3) differs from that of the wall of the segments themselves in the 

 following particulars : — 



(i) The wall is much narrowed down. 



(2) The connective tissue layer beneath the outer epithelial lining has disap- 



peared. 



(3) The inner thin transverse layer of muscles is absent. 



(4) A radial layer of muscle fibres pass from beneath the outer epithelial lining 



inwards to the epithelium lining the apertures. 

 The process of autotomy which occurs in the siphonal tube between the con- 

 tiguous segments thus seems to be due to the voluntary contraction of the radial 

 muscles which cut through all the other layers of the body-wall, thus separating one 

 or more distal segments from the proximal portion of the siphon. Even in the case 

 of spirit specimens the segments can be easily separated from one another. 



III. Foot. 



The foot is elongated and cylindrical, and is a little flattened from side to side ; 

 it is incapable of retraction within the mantle-lobes. The organ is stouter towards 

 the apex than towards the base, where it forms a distinct rounded annular swelling 

 and still further a conical process at the tip. When fully protruded , the foot has its 

 length a little less than that of the body (mantle-lobe) . There is a wide pedal-sinus 

 along the middle of the foot. 



IV. IyABIAL palps. 



The labial palps are shaped like an obtuse-angled triangle, the longest side of 

 which is a little curved and forms the lower border of the organ. The shortest side 

 of the triangle is attached to the side of the visceral mass at its junction with the 

 mantle-lobe. The measurements of the palp-margins (in a specimen 2*5 cm. in length) 

 are 0-45, 0-3, and o - 2 cm., respectively. 



V. Gills. 



The gills are narrow and elongated ; their posterior ends are slightly prolonged 

 into the base of the inhalent canal. The outer gill extends from the postero-inferior 

 angle of the labial palp ; the inner gill extends further forwards, and begins from 

 behind the postero-superior angle of the palp. This anterior portion of the inner 

 gill is overlapped by the palps. 



The attachments of the gills are best described in a table : 



