ANATOMY OF AKKNICOLA ASSIMTLIS. 743 



SetfB similai" to those above described are present in A, 

 marina^ and in some examples the Jamina is very well 

 marked, e. g. in a specimen of the Laminarian variety tlie thin 

 boi'der extends for nearl}' a millimetre along the seta, and 

 attains a width of 20 ft. Similar setae are present in A. 

 cristata, bnt in A. claparedii^ the lamina is not so well 

 developed, being short and narrow. In A. ecandata and A. 

 grnbii the lamina is also very narrow, seldom exceeding 

 abont 3 )u in width. 



The neuropodia of A. assimilis are easily seen, even in 

 the first segment. They are especially well developed in 

 the branchial region, where each resembles a pair of closely 

 applied tumid lips, between which is the row of ci-otchets. 

 These (fig. 9) are often curved, and ai-e O'G mm. to 0'7 mm. in 

 length, being considerably longer than chfetfe from specimens 

 of A. marina of the same size. (The longest crotchets found 

 in a specimen of A. marina 125 mm. long are only 0"47 mm. 

 in length.) The rostrum is short and blunt, even in unworn 

 clnette. There is a small subrostral enlargement, and about 

 six to nine teeth are present just behind the rostrum. 



Musculature. — The musculature calls for little comment ; 

 it is similar to that of A. marina, except that the oblique 

 muscles are present along the whole animal from the first 

 diaphragm to the end of the tail. They are exceedingly thin 

 bands, somewhat broader in the posterior part of the gill 

 region, but even here seldom exceeding 0'5 mm. in width, and 

 as a rule they are only 02 mm to 0*3 mm. wide. There is a 

 dorsal mesentery in the first and second chfetigerous seg- 

 ments supporting the dorsal blood-vessel. The three dia- 

 phragms are, as usual, situated at the anterior ends of the first, 

 third, and fourth chastigerous segments. There are no 

 pouches on the first diaphragm. This condition was con- 

 sidered to be so marked a feature of A. claparedii that it 

 was given as one of the diagnostic characters of this species 

 (Ga7nble and Ashworth, 1900, pp. 533, 541), since all other 

 Arenicolida3 whose anatomy is fully investigated possess 

 ' See Gamble and Asliworlli, 1900, pi. xxiii, lit,:. 23. 



