STOMACH AND INTESTINE. 



299 





rather suck the juices, of their casual or more 

 permanent victims. 



The simple digestive tube of the Acari or 

 Mites is prolonged in a straight line from 

 mouth to anus. It is sometimes complicated 

 by gastric caeca or dilatations. 



In the Aranei, or spiders proper, a slender 

 oesophagus passes back from the mouth to a 

 "stomach." This is sometimes a mere dila- 

 tation; sometimes is indicated by four sacculi, 

 that radiate from a narrow tube; and sometimes 

 presents a cavity, having blind prolongations 

 that extend into the bases of each of the 

 maxillary palpi and thoracic legs. All these 

 parts occupy the anterior or cephalo-thoracic 

 division of the body. The remainder of the 

 canal, entering the abdominal segment, dilates, 

 after a single convolution, into a large and 

 sometimes globular intestine, to reach the 

 anus by a short portion, of narrower diameter, 

 called a rectum. The long tubes met with in the 

 Insects recur in this order. One set, of vary- 

 ing size, open in the neighbourhood of the 

 complicated apparatus of prehension ; these, 

 from their position, are supposed to be sali- 

 vary. And occasionally a special poison 

 gland appears to empty itself in this neigh- 

 bourhood. A middle set, called hepatic, 

 often forms two pairs of tubes, with orifices 

 much posterior to the gastric sacculi ; in other 

 cases they are very numerous, and are con- 

 cealed by a granular mass, which occupies the 

 same situation. The posterior set are one 

 or two pairs of long caeca, which join the 

 intestinal cavity close to its termination, and 

 are hence compared to urinary organs. 



The Scorpions have a tolerably straight, 

 narrow, and simple tube, complicated by 

 several pairs of straight sacs, which come off 

 at right angles to its anterior part, and are 

 probably gastric crops. Below these, two 

 bifurcating tubes, of great length and small 

 diameter, open into a constriction of the 

 canal. They are regarded as hepatic. 



In the order of Mollusca, many of whom in- 

 habit the sea, we may again trace a gradual ad- 

 vance of development in the intestinal canal. 



The Tunicata is its lowest subdivision. 

 Here a simple canal begins by a wide oeso- 

 phagus, that leads from the bottom of the 

 branchial sac to a stomach or dilatation. This 

 is surrounded by a number of hepatic follicles, 

 that open into its intestinal end ; and it leads 

 to a wide recurved intestine, which ends by an 

 aperture on the upper and outer surface of 

 the animal. Sometimes the liver varies from 

 this description in the fact that its follicles are 

 aggregated. 



oO O 



The Bracliiopoda possess a digestive ap- 

 paratus of nearly equal simplicity with the 

 preceding. Dental structures are wanting ; 

 and the liver is still follicular. 



The Lamelfibranckiata exhibit a somewhat 

 similar condition. Their gastric cavity is 

 sometimes preceded by a short oesophagus. 

 From hence a comparatively simple intestine 

 continues, with a few convolutions, through a 

 mass of liver, to terminate, by a long straight 

 portion, in the anus. The latter segment, or 



rectum, lies along the hinge of their shell, and 

 often perforates the heart in its course. Al- 

 though the liver is large ami aggregate, it 

 opens by several ducts into the gastric dila- 

 tation.* 



The Gnxtcropoda have a head, jaws, and 

 salivary follicles. Their longer oesophagus 

 sometimes dilates into an ingluvies or crop. 

 Their stomach often possesses a thickened 

 lining, and a masticatory apparatus of teeth or 

 plates, which make it a kind of gizzard. 

 Sometimes it is divided into two or more 

 compartments. The large liver opens into the 

 pyloric extremity of the stomach, or the com- 

 mencement of the intestine, by one or more 

 ducts; or, rarely, it empties itself into the 

 oesophagus. One or two large glandular creca 

 also open into the beginning of the intestine, 

 and are regarded as a rudimentary pancreas. 

 The remainder of the tube is simple, and 

 ends anteriorly in the body, in accordance 

 with the general structure of the animal. In 

 the numerous herbivorous species, the intes- 

 tine is longer and more tortuous ; while the 

 crop, the gizzard, and the masticatory appa- 

 ratus all reach a high development. 



The intestinal canal of the Pteropoda is very 

 similar. It possesses jaws and salivary glands, 

 together with an oesophagus, a crop, a gizzard, 

 a short and simple intestine, and a conglo- 

 merate liver that often opens by a single duct. 



The Cephalopoda exhibit a marked advance 

 of development. Their mandibles form a 

 powerful organ of mastication ; and, in many 

 species, salivary glands co-exist. The mouth 

 leads to a long and dilatable oesophagus, 

 which descending, sometimes expands into a 

 crop before it finally reaches the gizzard or 

 muscular stomach. This organ is of tolerably 

 uniform appearance. Its shape is round, 

 or somewhat elongated ; it has a thick and 

 whitish epithelial lining, and its muscular 

 layer consists of two sets of fibres, each of 

 which radiates from a central tendon on one 

 side of the organ. The cardiac and pylo- 

 ric orifices are situated at its upper part. 

 The intestine coming from the latter soon 

 communicates with another cavity, which is 

 sometimes regarded as a stomach. This is, 

 in the lower Cephalopoda, nearly spherical. 

 But in many of the higher or Dibranchiate 

 division, it is of less simple form, being 

 triangular, elongated, or folded spirally like a 

 snail shell. Its mucous membrane is rugous 

 and follicular ; and the large liver, which is 

 still supplied by arterial blood, opens into it 

 by a single duct. The intestine continues 

 hence as an uniform tube, which, after one or 

 two slight curves, bends upwards to open at 

 the base of the funnel. In some species we 

 also find caecal appendages, the ducts of which 

 join those of the liver before they enter the 



* In some species a curious style or hard conical 

 process occupies a tube of similar shape, that com- 

 municates with the gastric dilatation. The use of 

 this implement is unknown ; but it has been sug- 

 gested to effect a triturative process: a supposition 

 which, if true, would render the cavity containing 

 it the analogue of a gizzard. 



