Studies on The Comparative Physiology of Digestion

Abstract

ABSTRACT Nephrops’ norvegicus is a marine Decapod Crustacean living at a depth of between 10 and 60 fathoms and never coming inshore. It lives principally upon a calcareous and carnivorous diet. Food is seized by the great chelae or second and third walking legs and passed to the third maxillipedes, which grip and tear it with their teeth and transfer it to the preceding mouth parts. It finally passes to the mandibles which grind it up still finer, and pass it into the mouth. The alimentary system consists of a chitinous fore-gut, comprising an œsophagus, a cardiac fore-gut, and a pyloric fore-gut, a mid-gut which is the longest part of the gut and possesses three outpouchings--the dorsal cæcum, the posterior diverticulum, and the hepatopancreas which opens by a pair of lateral ducts into its anterior end-and a chitinous hind-gut which opens to the exterior at the anus. Tegumental glands are found in great numbers in the mouth region, the œsophagus, and the hind-gut. There is no physiological evidence to show that they possess any digestive function; they may possibly discharge a sticky secretion or be correlated with the presence of chitin in these regions. The cardiac fore-gut consists of a spherical bag strengthened by calcified ossicles, a number of which form the gastric mill, a masticatory apparatus consisting of three teeth and worked by the gastric muscles. The pyloric fore-gut is separated from the cardiac fore-gut by the cardio-pyloric valve. Finely divided particles pass into it by the mid-gut filter, and by the two lateral channels. Behind the valve the walls of the pyloric fore-gut are thickened to form the press and beneath this lies the gland filter, consisting of two chitinous plates, uniting in the middle line to form a ridge, and bearing transverse chitinous rods each of which is set with a row of setae on its inner side. Five chitinous valves pass backward into the mid-gut. The hepatopancreatic secretion passes directly into the cardiac fore-gut by way of the gland filter and two ventral channels, one on either side of the cardio-pyloric valve, which open into a ventral groove in the cardiac fore-gut. Material dissolved by the secretion is passed back by the same route, being strained through the gland filter before entering the hepatopancreas. The epithelium of the hepatopancreas is composed of glandular ferment cells and absorption cells which contain a mass of fat globules. The tubules are surrounded by a network of circular and longitudinal muscle fibres. The mid-gut possesses a circular muscle coat and outside that a longitudinal one, but the hind-gut also possesses inner longitudinal fibres, which run within the six longitudinal ridges into which the hind-gut epithelium is raised. The anus is a longitudinal slit on the under side of the telson. It possesses no sphincter but a series of radial muscles which pass to the dorsal and particularly the ventral body chitin. Food is forced into the cardiac fore-gut by the action of the constrictor and dilator muscles of the œsophagus. There it is either ground up by the gastric mill and passed into the mid-gut, or dissolved by the digestive juices and passed into the hepatopancreas, the expansions and contractions of which are brought about by its muscular network. Peristaltic action forces particles backward in the mid-gut, and pronounced peristaltic contractions, due to the action of the inner longitudinal muscles, force it out of the hind-gut, the anus opening by a contraction of the radial fibres. The hind-gut is innervated from the last abdominal ganglion by the nervus intestinalis posterior, which probably regulates the peristaltic movements. The hepatopancreas secretes the digestive enzymes. Starch, glycogen, sucrose, maltose, and lactose are digested, the amylolytic ferment finding its optimum in a neutral medium and at a temperature of 57° C, being destroyed between 76° and 780 C. Fat and esters are split up ; and protein is digested, especially in alkaline media, with the formation of amino-acids. There is no bile, but a brown lutein pigment is present in the hepatopancreas. The accessory functions of excretion, elimination, and regulation are all of doubtful occurrence. Absorption is carried on by the mid-gut and its appendages, and by the absorption cells of the hepatopancreas. The fore-gut (and probably the hind-gut) is a semipermeable membrane. The mid-gut allows dissolved matter to diffuse through it in either direction, but the action of the absorbing epithelium causes contained fluid to pass outward even against strong osmotic pressure. Among the Crustacea the hepatopancreas is an important storage organ for glycogen, fat (including lecithin), and calcium, which vary in amount in accordance with the particular stage of growth. Nephrops stores fat and glycogen but not calcium.