126 A. J. CARLSON. 



Quatrefages describes two pairs of exceedingly small ganglia situ- 

 ated in the posterior part of the pericardial cavity. Nerves from 

 these ganglia reach the base of the auricles and enter the muscu- 

 lature of these organs. Other branches from the ganglia ramify 

 in the pericardium. A small nerve connects each pair of ganglia 

 with the large pleuro-visceral ganglion on the ventral surface of 

 the posterior adductor muscle. Hancock and Embleton describe 

 " two small elliptical ganglia attached to the anterior and under 

 part of the branchial (visceral) ganglia and united by commissures. 

 These send filaments to the ovary and to the ventricle of the 

 heart." The ganglia and the nerves are not figured, nor is it 

 stated in what way the nerves reach the heart. List (1902) in 

 his recent monograph of the Mytilidce of the Gulf of Naples makes 

 no mention of the innervation of the cardiac apparatus. He de- 

 scribes nerves to the renal organs and the reproductive glands, 

 but follows them only a very short distance from the pleuro-vis- 

 ceral ganglia. Dogiel (1877) has described nerve cells in the 

 auricles and at the auriculo-ventricular junctions of Pecteti and 

 Anadonta, but he does not make out their connection with any 

 nerves and ganglia outside the heart. 



My own work was done on Mytilus calif or niamis, Mya arcnaria, 

 Tapes staininea and Platydon cancellatus. In Mytilus (PI. IV., Fig. 

 i) I found a series of small ganglia that probably correspond to 

 the cardiac ganglia described by Quatrefages in Tcrrcdo. A small 

 nerve is given off from the dorsal side of each visceral ganglion 

 {?'). These nerves (4) run dorsally for a short distance close to 

 the anterior surface of the posterior adductor muscle and then 

 turn anteriorly, reaching the dorsal body wall through a portion 

 of the reproductive gland. At this point two slender branches 

 are given off; the lateral branch (5) can be followed anteriorly in 

 the body wall into the base of auricle, but it cannot with certainty 

 be traced on to the auricular musculature. The other nerve (6) 

 can be followed anteriorly on to the pericardium near the median 

 line. The main branch of the nerve takes a median course in 

 the body wall towards the median line, but before reaching the 

 median line it bifurcates, one branch passing posteriorly (7) and 

 one anteriorly in the dorsal body wall. At the point of bifurca- 

 tion there is a small ganglion (8, 9). The branch that takes the 



