ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 1 59 



2. A large optic nerve (?'). 



3. A nerve to the tentacle. 



4. A small nerve from the under surface, which terminates in the 

 auditory sac (7). 



5. A large and long commissural branch which runs backwards 

 and downwards past the stomach, to unite with the pedal ganglion of 

 its side. 



The Pedal Ganglia (jj/) are two large ovoid masses in contact by 

 their inner edges. They give off — 



1. Two branches downwards to the propodium. 



2. Two branches upwards to the dorsal parietes. 



3. Two large branches, which run at first separately below the 

 stomach, and then unite to form a single trunk. This runs along the 

 stomach and intestine, sometimes twisting round them and giving off 

 branches to them and to the parietes ; and on the intestine it sepa- 

 rates again into two chords, which join two small ganglia (3) placed 

 between the aorta and the intestine ; one lies on the aorta, the other 

 between this vessel and the intestine, and they are connected by a 

 commissure. From the former of these ganglia, which is the smaller, 

 two nerves pass upwards and join a flatfish mass placed immediately 

 beneath the " subspiral ciliated band." There was an obscure ap- 

 pearance of branches radiating from this mass, and it is probably 

 ganglionic. 



Organs of Sense. The Eyes (fig. 8). — The eyes are very perfectly 

 organized ; each eye is contained within a chamber, excavated in a 

 papilla, whose convex wall forms a sort of supplementary cornea, 

 answering to the cornea of Cephalopoda, or to the corresponding 

 cutaneous cornea of the Gasteropoda. 



Their eye-proper is suspended within this chamber by a number of 

 irregular muscular bands, which stretch from it to the walls of the 

 chamber, and perform the function of oculi-niotores. The optic nerve 

 penetrates the inner wall of the chamber and enters the eye from that 

 side. 



The eye-proper is elongated and somewhat hour-glass-shaped, 

 being contracted just behind the crystalline lens. The constriction 

 divides the eye into two portions, an internal and an external. The 

 latter is almost spherical, and is formed by the true cornea, which is 

 much thicker in the middle than elsewhere, so as to present a meniscus 

 section. Behind, the cornea is continuous with the sclerotic coat, 

 which is thick, and seems to be continuous with the neurilemma of 

 the optic nerve. 



The crystalline lens is spherical, and is separated by a very small 



