SENSE ORGANS. 127 



which are difficult to determine. The anterior aorta 

 runs through the centre of the mass, marking the 

 division between the antero -inferior portion, or pedal 

 ganglia, and the postero-superior portion, or viscero- 

 pleural ganglia. 



a. The pedal ganglia are a pair of closely apposed 



masses, from which a number of large nerves 

 pass to the foot. 



b. The viscero-pleural ganglia consist of at least two 



pairs of ganglionic masses, closely apposed to each 

 other and to the pedal ganglia. From them large 

 nerves pass outwards to the viscera and to the 

 body-wall. 



3. The structure of the ganglia. 



Remove one of the ganglia ; place it in 1 per cent, acetic 

 acid for a short time to soften the connective tissue, and then 

 tease it on a slide in glycerine. Cover ; and examine it with 

 low and high powers. 



The nerve-cells are large granular cells with very 

 large and distinctly reticulate nuclei, and with long 

 branching processes by which they are connected 

 with one another and with the nerve-fibres. 



VI. THE SENSE OEGANS. 



1. The eyes are placed one at the end of each of the larger 



tentacles, and are only exposed when these are fully 

 protruded. Each receives a branch from the large 

 nerve supplying the tentacle. 



Each eye is a small spherical vesicle, the wall of 

 which is strongly pigmented in its posterior portion, 

 and lined by a layer of retinal cells. The cavity of 

 the vesicle is partially filled by a large cuticularlens, 

 which projects into it from its anterior wall. 



2. The ' auditory ' organs are a pair of small spherical sacs 



imbedded in the substance of the pedal ganglia, 



