790 PROF. G. C. BOURNE ON THE 



of them are the otocysts, ot. ; ventrad of them is the pedal gland. 

 Each cord consists of a core of nerve-fibres and dendrites surrounded 

 by a cortical layer of nerve-ganglion cells. The latter send in 

 lateral horns at about the middle of the outer side of each cord 

 in such a manner as to divide the central core into upper and 

 lower moieties, which have been identified by French authors 

 with the pleural and pedal sections of the cords respectively. 

 This interpretation, however, does not appear to me to be well 

 founded. Fig. 46 represents a section somewhat fuither 

 forward than that in fig. 45 ; it passes through the hinder part 

 of the anterior pedal commissure, and includes the roots of the 

 two nerves of the pedal gland and of the right opercular nerve. 

 The nerve-fibres of the former are seen to be supplied from two 

 areas of the cortical layer lying respectively on the outer and 

 inner sides of the ventral side of the cord. The opercular nerve 

 receives its fibres pai-tly from a centre on its own side, partly 

 from a centre on the opposite side of the cord, the latter fibres 

 crossing over in the commissure. Below them is a stout band of 

 commissural fibres connecting the lateral horns of nerve-ganglion 

 cells with one another. Above the median ventral raphe is a 

 thick mass of nerve-ganglion cells. In a section somewhat 

 further forward (fig. 47) the ventral raphe has disappeared and 

 the mass of ganglion- cells above it is only represented by two 

 small islets of nerve-ganglion cells, which are separated from the 

 ventral surface by two well-defined bands of nerve-fibres start- 

 ing from the ventro -lateral groups of nerve- ganglion cells and 

 passing towards the centre partly decussate, partly sweep round 

 the islets to curve rovind to the lateral horns of the ganglion- 

 cells. Above these curved bands is the well-defined transvei-se 

 commissural band of nerve-fibres, and above this again there is 

 on/ either side a centre, consisting of nerve-fibi-es overlaid by a 

 layer of ganglion-cells, which is seen to be connected with the 

 origins of the opercular nerves. Above the nervous mass a pair 

 of muscular cords passing from the otocysts towards the centre 

 should be noticed. 



In the next section (fig. 48), taken some little way further 

 forward, the two little islets of ganglion-cells lying opposite the 

 lateral horns are still visible. Below them are transverse bands 

 of commissuial fibres. Between them is seen the most anterior 

 part of the decussating tract observed in the previous section. 

 After decussation the fibres sweep out right and left to the dorso- 

 lateral regions. In the mid-dorsal line is a deep and wide groove 

 into which the muscular cords noted in the last section are 

 entering. I regard the whole of the sections hitherto described 

 as belonging to the pedal centres. The next section (fig. 49) 

 shows that the dorsal groove containing the two muscular cords 

 has been converted into a canal by the upgrowth and dorsal 

 union of the nervous tissue. All that lies below this canal 

 belongs to the pedal centres ; all that lies to the sides of and 

 above the canal belongs to the pleural centres. 



