Fresh-water Mussel. 63 



anteriorly to that of the protractor pedis. Three commissural cords, 

 under each of which a slip of blue paper has been placed, pass off 

 from it. The one which passes vertically downwards in the pre- 

 paration, and passes vertically, or nearly so, upwards in the position 

 ordinarily maintained by the animal during* life, brings the labial 

 gaug-lion into commissural junction with the parieto-splanchnic or 

 branchial gang'lion, which is seen lying upon the posterior adductor. 

 A second cord passes obliquely forwards towards the foot, to join 

 the pedal ganglion, which is situated a little way within the in- 

 ferior periphery of the ' visceral mass,' a division of the body which 

 in these molluscs is less sharply differentiated from the * foot' 

 proper than in many other members of the class. The third cord, 

 which looks like a production of the second, from the ventral to 

 the haemal side of the plane of the labial ganglion, is in reality the 

 cord of commissure between this ganglion and its fellow of the 

 opposite side. This cord holds the same relation to the commence- 

 ment of the digestive tract as the commissure of the cephalic 

 ganglion holds in the Odontophora ; and the cord of commissiire to 

 the pedal ganglion is, as in them, placed anteriorly to the cord 

 of commissure to the parieto-splanchnic. There are, however, no 

 stomato- gastric, nor any separate sympathetic ganglia in the La- 

 mellibranchiata. The absence of the former of these structures is 

 obviously correlated with the absence of any prehensile apparatus, 

 or any triturating ' buccal mass,' whilst the absence of the second is 

 ordinarily to be noted in structures and in organisms, which are as 

 richly provided with cilia as are those of this class. Special siphonal 

 ganglia are however sometimes superadded to the three pairs of 

 ganglia here specified in the siphonate species ; and small ganglia 

 may be developed in this class along the free edge of the mantle^ 

 in connection with the sensory plexuses which the anterior and 

 posterior pallial nerves, from the labial and parieto-splanchnic 

 ganglia respectively, make up by their ramifications along it. The 

 cord of commissure from the labial to the parieto-splanchnic ganglia 

 passes from before backwards ; firstly, between the fibres of the 

 anterior retractor and those of the protractor pedis ; and, secondly, 

 after skirting the orifice of the reiDroductive gland, on its inferior 

 or inner edge, through the glandular portion of the organ of 

 Bojanus, externally to the tendon of the posterior retractor. Im- 

 mediately posteriorly to this tendon, and anteriorly to the large 



