89 



surface formiuff the cortex. The anterior central lobes 

 are almost entirely composed of ganglion cells, which also 

 abound on the central part of the dorsal surface (that is, 

 the surface against the adductor muscle). The main part 

 of the ganglion is made up of fibres, amidst which can be 

 seen a very definite transverse series ; probably the repre- 

 sentative of the visceral commissure. 



SENSE OEGANS. 



Pecten is unusually well supplied with sensory struc- 

 tures, certain of which, the pallial eyes, attain a high 

 degree of specialisation, and are a remarkable feature of 

 the animal. 



The sense organs are of five kinds, the first three, and 

 probably the fourth, of which are common to the Lamelli- 

 branchiata, and do not differ to any great extent in the 

 various genera. They are: — (a) Sensory cells in the 

 epidermal layer ; (b) a pair of otocysta ; (c) a pair of 

 osphradia ; (d) an abdominal sense organ ; (e) a series of 

 highly-developed eyes. 



(a) The Sensory Epithelial Cells. — These, the 

 " Pinselzellen " of Flemming (30), are to be found 

 scattered all over the epidermis, but are present in greatest 

 numbers on the long, extensible tentacles of the mantle 

 edge, where they occur between two " Stiitzzellen," which 

 according to Rawitz have a common membranous 

 covering. They are most abundant near the apices, and 

 render each of these tentacles a most important tactile 

 organ. This view is confirmed when the animal is 

 observed living, and the greatly extended tentacles can be 

 seen moving slowly to and fro in the water. A ver}' slight 

 motion in the water appears to be stimulus enough for a 

 sudden retraction. 



