74 COMMISSION OF CONSERVATION 



The Nervous System of the youngest stage of the spat still retains 

 cerebral, pedal, and visceral ganglia, but the two former are already be- 

 coming difficult to recognize, while the latter are more evident than in the 

 larva. The cerebral and pedal ganglia disappear with the velum and foot, 

 whose purposes they serve — first the ganglion-cells and then the nerve 

 fibres becoming reduced. The visceral ganglia on the contrary persist and 

 become conspicuous from their size, symmetry, and position in front of 

 the great adductor muscle (Plate V, fig. 33). They preserve a distinctly 

 paired structure (Plate VI, fig. 15; Plate VII, figs. 6, 7) and have a central 

 nerve mass surrounded by a layer of more deeply staining ganglion-cells. 

 The limitation of the foot to a brief transitory existence during the free 

 life of the oj^ster will explain the atrophy of the pedal ganglia. Fixation 

 and consequent loss of activity, together with a shifting of responsibility 

 to the gills, and the return to a degree of radial symmetry, have dispensed 

 with the need for cephalic ganglia, and thrown everything within the 

 range of the visceral ganglia. Two large nerves can be traced forwards, 

 in 1 mm. spat, from the ganglia, one on each side of the descending lobe 

 of the stomach, for fully two-thirds the distance towards the mouth. 



Cephalic ganglia have been mentioned (and even figured) for the adult oyster by 

 Ryder, Hyatt, Grave, and others, but, despite careful search through sections of different 

 sizes of spat as well as dissections of large spat and adults, I have never been able to find 

 them, and I fear they have been imagined by analogy with other genera in which they 

 are easily found (Mya, Mactra, Venus, Mytilus, Anodonta, Cyclas, &c.) 



Sense-organs such as eye-spots and otocysts, useful during the active 

 life of the larva, disappear rapidly upon the assumption of the quiescent, 

 involuntary existence of the spat. High epithelium occurs at the margin 

 of the mantle behind the adductor muscle, in early spat stages. 



A Heart having a thin wall and containing large, deeply staining, 

 nucleated blood-cells, is situated in front of the posterior adductor muscle 

 and visceral ganglia. In the recently attached spat it occupies the space 

 between these and the stomach in front, and with the two lobes of the 

 liver on the sides (Plate VI, fig. 15). In 1 mm. spat, where the adductor 

 muscle has moved downwards and the stomach become more developed 

 and rotated backwards, the heart lies rather above the adductor muscle 

 and below the rectum. 



Nephridia (or what I take for them) are to be found in spat of '79 mm. 

 and larger (Plate VII, fig. 6), as two tubes in the angles close against the 

 adductor muscle, and running forwards on each side of the visceral 

 ganglia. They are relatively large, hollow tubes, composed of pale, 

 nucleated, epithelial cells. They occur in all sizes of spat, but do not re- 

 tain the simplicity of the younger stages, being represented in the larger 

 ones by several tubes side by side or branching, some of which extend far 

 forwards to the region of the genital organs and suggest that they may 

 later serve as genital duct. Some of them again extend far backwards 



