23 



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. Hound 

 the sac is a layer of connective tissue, outside which can 

 be seen the muscles of the mantle (fig. 15, M X M.). 

 According to Appellof , 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 Eledone, 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 

 Eledone still has are most efficient — powerful suckers, 

 great 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. Eledone, like Octopus, 

 shows this dorsal fusion in a more complete stage. A thin 

 sheet of muscle is continued anteriorly from the dorsal 

 edge of ihe 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. 



