SEA-FISHERIES LABORATORY. 421 
surface forming 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 1s, 
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 ORGANS. 
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 otocysts; (¢) a pair of 
osphradia; (d) an abdominal sense organ; (¢) a series of 
highly-developed eyes. 
(a) THe Sensory Hrrruentan Criis.—tThese, the 
“Pinselzellen”’ of Flemming (80), 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 froin the water. A very slight 
motion in the water appears to be stimulus enough for a 
sudden retraction. 
