THE SENSORY ORGANS. 63 



The pedal ganglia, in Ct/c/a# and the Unionidae, according to the 

 somewhat similar accounts of ZIEGLER and F. SCHMIDT, with which 

 also that of ScHiERHOLZ can he harmonised, in their formation are 

 associated with the byssal gland. Shortly before the paired byssal 

 gland becomes invaginated (Fig. 19, p. 40), at the point where it is 

 to form, a number of cells become detached from the ectoderm. 

 These at first lie beneath the floor of the invagination, but then 

 separate from the latter and shift further forward, at the same time 

 coming closer together, forming the rudiments of the pedal ganglia 

 (Fig. 21 B, p. 44, and Fig. 31, p. 75). 



In TV/Wo, the pedal ganglion, according to HATSCHEK, arises as an 

 ectodermal thickening even before the foot begins to form (Fig. 18, 

 ij, p. 36). It occupies at first a large part of the ventral surface, 

 but appears to decrease in size after its detachment from the ecto- 

 derm. During its severance, the mesoderm grows round it. The 

 division into two parts is not so distinct here, but is indicated by a 

 median line. The two halves of the ganglion 'are thus in this case 

 connected from the first. When the foot rises up and grows out on 

 the ventral side of the larva, the ganglion remains lying at its base. 



In their manner of formation the visceral ganglia agree closely with 

 the cerebral and the pedal ganglia. They arise in the groove 

 between the gills and the body, almost at the posterior end of 

 the foot. 



The cerebro-visceral [pleura-visceral] commissure has its origin, 

 according to ZIEGLER, in a cell-strand which becomes detached from 

 the ectoderm in the groove between the gill and the body, and runs 

 forward from the visceral ganglion, and later becomes a commissure. 



[In Nucula and the Protobranchia generally, distinct pleural 

 ganglia are present. These are situated immediately behind the 

 cerebral ganglia at the commencement of the visceral commissures ; 

 here, also, the pleuro-pedal commissures are for some distance in- 

 dependent of the cerebro-pedals. In other Lamellibranchs, the 

 pleural ganglia are fused with the cerebral. DREW was, however, 

 unable to trace a distinct origin for the pleural ganglia in Yoldia.] 



C. The Sensory Organs. 



The Eyes. It may be stated with some certainty that the 

 simply constituted eyes of the border of the mantle, i.e., the so-called 

 invaginations, or optic pits, and the compound eyes arise through a 

 comparatively slight differentiation of the mantle-epithelium. 



