Some aspects of the anatomy of the alimentary canal of the lesser octopus, Eledone cirrhosa (Lamarck 1817), with special reference to the nervous system

Abstract

The modern cephalopod molluscs, particularly the octopods, are highly developed macrophageous predators which have a pronounced ability to learn. Striking resemblances are found between these animals and the vertebrates, particularly the fish. The study of cephalopods therefore provides an opportunity to examine systems which are composed of structural units common to molluscs but which may be similar in function and performance to those of vertebrates. This investigation concerns the alimentary canal and its innervation in the lesser octopus Eledone cirrhosa.

Detailed anatomical investigation shows the alimentary canal of E. cirrhosa to be very similar to that of the better known Octopus vulgaris. Current concepts of the functions of the alimentary organs are discussed utilizing data from both E. cirrhosa and O. vulgaris.

Silver staining reveals a basic pattern of innervation in the alimentary organs. Large nerve trunks running in the external collagenous layer give rise to a nerve plexus within the circular muscle. The longitudinal muscle plexus arises from branches of the circular muscle plexus or direct from the nerves of the external layer, Nerves of both plexuses contact muscle fibres in an 'en passant' manner. Fibres run out from the longitudinal muscle to the subepithelium, where they are observed associated with muscle fibres and beneath the epithelial basal lamina. Good evidence for fibres crossing into the epithelium was observed only in the cuticularized regions of the digestive tract. The digestive gland ducts differ from this pattern in the very large numbers of major nerves seen in their external and muscular regions. Outwith the alimentary ganglia nerve cells are only regularly observed within the major intestinal nerves. Receptor like cells were also repeatedly observed only in the posterior intestine. These results axe compared with data from O. vulgaris and the physiological evidence for the presence of receptors. The blood vessels of the alimentary canal are innervated at all levels. The distribution of other densely staining cells is reported.

Evidence for the presence of particular neurotransmitters within the alimentary canal and alimentary nerve centres is reviewed. Fluorescence histochemistry shows that at least two types of nerves are present in the alimentary wall. The majority axe aminergic (including those associated with blood vessels and some sphincters), as the pattern of fluorescent nerves is predominantly that shown by silver studies. However, fluorescent nerves decrease and then disappear anteriorly from the crop/oesophageal sphincter and posteriorly from the mid-intestine. The stomach has fluorescent nerves, other than those associated with blood vessels. Fluorescent fibres enter the gut via the sympathetic and possibly the digestive gland duct nerves. Non-fluorescent fibres enter via the atrio-rectal nerves and from the gastric ganglion. The fluorescence is ascribed to catecholamines as no evidence of 5HT was obtained, Specific fluorescence was also observed in some cells of the subepithelium and the external region. These results are discussed with reference to available physiological data.

The anterior intestine was the representative region chosen for fine structural studies. These show the alimentary muscles to be the same basic type (cross or pseudo-striated) as that found in cephalopod somatic or heart muscle. Three types of myomuscular and neuromuscular junctions axe described. The presence of mineralized concretions (spherites) in the external layer of the intestine, together with the complex relations of its epithelial cells and heavy vascularization suggest a secondary function of mineral and/or water balance for this organ.

These results are discussed.

Finally, a pathological condition affecting the octopuses during this study is described.