THE COMMON MUSSELS. 35 



oblique vein does not lie in any large passage or " couloir " (o v, 

 Fig. 33) ; and the floor of the pericardium, so far as I can find, is 

 intact except where it is pierced by the oblique vein. The substance 

 of the organ of Bojanus surrounds the oblique vein right up to the 

 floor of the pericardium, but there is no communication between the 

 pericardium and it round the neck of the oblique vein. However, 

 the oblique vein in M. latus has a different position in relation to the 

 posterior retractors from what is the case in M. edulis, and the posi- 

 tion in M. latus corresponding to that of the " couloir " in M. edulis 

 would be just in front of the middle retractors of M. latus (compare 

 Figs. 33 and 34). But there is no communication between the peri- 

 cardium and organ of Bojanus in front of the middle retractors, 

 although the cavity of the pericardium extends into two blind pro- 

 longations above these muscles (Fig. 33). In M. latus I have been 

 unable to find, either by injection or dissection, any communica- 

 tion between the organ of Bojanus and the pericardium ; and, since 

 such a passage is so easily seen in M. edulis, I am inclined to think 

 that the internal opening of the renal organ is lost in M. latus. In 

 that case the organ of Bojanus will have departed further from the 

 primitive condition or become more modified in M. latus than in 

 M. edidis or magellanicus. 



Nervous System. 



[I omit here part of the original paper containing a short historical 

 account of our knowledge of the nervous system of Lamellibranchs.] 

 The nervous system of M. latus consists of three pair of ganglia, the 

 two hindmost pair being each connected with the anterior pair by 

 stout nerve-cords. The anterior ganglia have been termed the cere- 

 bral, anterior, or buccal ; the median ganglia the pedal or median ; 

 and the posterior ganglia the visceral, posterior, or parieto-splanchnic. 

 In each case I shall use the first as here given — namely, cerebral, 

 pedal, and visceral {cg,p g, and vg, Figs. 1 and 17). In the plate 

 of the nervous system (Fig. 17) it is seen that the ganglia in each 

 pair are connected with each other as well as with the ganglia of 

 another pair. It has been recommended that the term " commissure " 

 be employed to denote the connecting nerve-cord between two ganglia 

 of one pair, while the term " connective " should be applied to the 

 nerve-cords connecting ganglia of different pairs. Using these terms, 

 then, the main part of the nervous system of M. latus consists of 

 three pair of ganglia — the cerebral, the pedal, and the visceral; of 

 three commissures — the cerebral, the pedal, and the visceral ; and of 

 two pair of connectives — the cerebro-pedal and the cerebro-visceral. 



