ELEDONE. 491 
figure also shows that the rods are built up of concentric 
layers of chitin, the innermost, and therefore oldest, 
layers staining most deeply. Among the chitinous layers 
may be noticed a few cells with deeply staining nuclei. 
These are probably degenerate cells from the epithelial 
sac, which have become surrounded by chitin. Round 
the sac is a layer of connective tissue, outside which can 
be seen the muscles of the mantle (fig. 15, @,M.). 
According to Appelléf, the epithelial sacs in Octopus, and 
therefore probably in Eledone, are formed by the shell 
gland. This gland, after closing and sinking below the 
external surface of the mantle, divides into two halves, 
each of which takes up a lateral position and secretes the 
stylet of its side. Hence these stylets represent the shell 
in Eledone, i.e., they are the homologues of the shell of 
other Mollusca, although much reduced in size and 
importance. The fact that the great muscles of the 
funnel, cephalopedal mass, and the muscles of the mantle 
radiate from these stylets, also gives support to this view. 
Possibly this degeneration of the shell in Hledone, as in 
other Octopoda, may be explained by the fact that it is no 
longer needed as a means of protection. For we must 
recognise that the means of offence and defence that 
Hledone still has are most efficient—powerful suckers, 
ereat biting jaws, immense bodily strength, together with 
the ink sac and large far-seeing eyes. 
Dorsal fusion of head and mantle.—In the Decapoda 
the head and visceral dome are not as a general rule 
united dorsally. However, in Sepiola there is a narrow 
connection between the two. LEledone, like Octopus, 
shows this dorsal fusion in a more complete stage. A thin 
sheet of muscle is continued anteriorly from the dorsal 
edge of the mantle over the region of the eyes, and fuses 
with the muscular bases of the arms, thus forming a firm 
dorsal union between the head and visceral dome. 
