POLYCH^TA SEDENTARIA OF THE FIRTH OF FORTH. 649 



Habits. — Very common at Gran ton in flats of somewhat muddy sand on 

 shore, and beyond low-water mark. The sides of its burrows are stained with 

 the yellowish-green exudation from its body. The intestine is always full of 

 the sand and mud in which it lives, and from which it derives its entire nourish- 

 ment. This sand is ejected in a cylindrical rod from its anus, and this forms 

 a spiral coil on the surface of the shore: near the "cast" is usually a wide hole, 

 from which the head is protruded when the tide is up. The worm is usually 

 found at least a spade's depth from the surface when the tide is out. 



Anatomy. — The 1st somite is destitute of bristles ; the following 6 bear 

 each a dorsal fascicle of hair-like bristles and a ventral torus uncinigerus, but no 

 branchiae. The following 13 bear both fascicle and torus, and in addition a pair 

 of plumose branchiae ; the rest of the body, which is variable in length, is 

 thinner than the rest, and has neither fascicle, torus, nor branchiae, but is 

 cylindrical, and covered uniformly with papillae. This part forms the caudal 

 appendage : the number of somites in it we have not determined. 



On dissection, it is found that the body-cavity is a large and well-defined 

 space in the anterior and branchial regions, but almost obliterated in the 

 caudal appendage. Transverse mesenteries are only present in the anterior 

 4 somites, and here they are incomplete ; in the rest of the body they are absent. 

 There is a septum between the buccal and the 1st chaetiferous somite, none 

 between the 1st and 2nd chaetiferous ; one behind the 2nd, and one behind the 

 3rd chaetiferous. 



Between two successive parapodia are seen externally 5 constrictions, of 

 which the 5th is the deepest. Between the 5th and the parapodia is a pro- 

 minent ring ending in a sharp line. The mesenteries are attached to the body- 

 wall at the 2nd constriction. There are 6 pairs of nephridia, visible in spirit 

 specimens as brown glandular masses attached to the body- wall ; they belong- 

 to somites 5-10. Claparede, in his description of Arenicola Grubii, describes 

 only 5 pairs of nephridia, belonging to the 4th to the 8th setegerous somites ; 

 that is, somites 5-9. He does not mention any difference in the case of A. marina. 

 In the latter the first pair of nephridia is smaller than the succeeding, and is to 

 that extent rudimentary. The internal opening of the 1st segmental organ is 

 on the anterior face of the mesentery, between somites 4 and 5. 



The nephridium is a wide thin-walled tube, showing on dissection a black 

 colour, which is due to the concretions in the glandular cells lining the cavity. 

 The peripheral end of the tube opens to the exterior by a pore visible in the 

 fresh animal to the naked eye, and situated immediately behind the dorsal 

 extremity of the torus uncinigerus. Anterior to the pore in a living uninjured 

 animal the black tube of the nephridium is seen through the semitransparent 

 skin, and is very conspicuous. The peripheral end of the tube, which is posterior 

 in position, is somewhat dilated, and lighter in colour than the rest. Anterior 



