THE STOMACH IN ANNULOIDA. 591 



large intestine. Certain fine csecal tubes communicating with it, are probably 

 glandular structures rather than diverticula of the intestine. The first part 

 is undoubtedly fitted for absorption, whilst the lower end is more excretory. 

 It presents a terminal dilatation or cloaca, into which the reproductive organs 

 open. 



In the Myriapods, the alimentary canal is narrow and nearly straight, and 

 is either, as in the carnivorous species, merely slightly dilated, to form a 

 stomach, or, as in the vegetable-feeders, complicated by pairs of saccular pro- 

 jections, which have been regarded as crops, or gizzards, but may be merely 

 glandular recesses. The intestine is straight, wide, plicated, and sometimes 

 sacculated. Ceecal tubuli open into various parts of the alimentary canal. 



In the Arachnida, the digestive tube is straight, very short, and compara- 

 tively simple. The stomach, scarcely dilated, has sometimes four appended 

 sacculi, and sometimes csecal prolongations, reaching into the bases of the 

 palpi and legs. The intestine sometimes presents a globular dilatation, before 

 it finally narrows. 



Amongst the Crustacea, the higher forms, such as crabs and lobsters, pos- 

 sess a short wide sac, provided with internal hard calcareous, or chitinous 

 denticles, which serve at once the purpose of a gullet, a masticatory apparatus, 

 a stomach, and a gizzard. The denticles, arranged symmetrically around the 

 canal, are worked by powerful muscles, and are shed when the animal changes 

 its shell ; besides the larger denticles, there are often stiff hairs, bristles, and 

 horny ridges. The intestine, marked off by a constriction from this denticu- 

 lated stomach, is short, nearly straight, and simple ; it is sometimes subdi- 

 vided by an imperfect valve, and, though seldom, has one or two cseca. In the 

 lower parasitic Crustaceans, the alimentary canal is, however, straight and 

 simple, becoming narrower as it passes backwards. 



The shortness and simplicity of the alimentary canal in the Spiders, Scor- 

 pions, and Crustacea, which live, some upon the juices of other animals, and 

 some on crushed animal food, compared with the length and complexity of the 

 digestive tube in the vegetable-eating insect larvse, or in the perfect beetles, 

 further illustrates the modifications already noticed in the digestive canal of 

 the higher animals, according to the nature of their food. 



In the Annelida, the alimentary canal never presents any convolutions or 

 bendings, and the mouth and outlet are always at opposite ends of the body. 

 It has no mesentery. It is either quite simple, not even presenting a gastric 

 dilatation, as in the lower marine species, or it is developed into simple tubuli, 

 or subdivided pouches, or it may be regularly sacculated, as in the leeches, 

 the blood sucked by those animals being retained, and slowly digested in the 

 sacs. In the earth-worm, these sacs are represented by simple constrictions ; 

 it also has a sort of gizzard, and, within the intestine, a tubular csecal organ, 

 named the typhlosole, the use of which is not known. 



Annuloida. In the liotiferous animalcules, the alimentary canal presents a 

 pharyngeal dilatation, or crop, sometimes regarded as the stomach ; the intes- 

 tine is narrow and simple, opening sometimes at once on the surface, some- 

 times after forming a sort of cloaca ; the orifice is usually near the hinder end 

 of the body, on its dorsal aspect. In the Turbellaria, minute marine and fresh- 

 water worms, an alimentary canal is present, which is either simple, sacculated, 

 or most remarkably ramified or dendritic ; with few exceptions, such as the 

 I^emertis and Microstoma, it has but one aperture, viz., a mouth, which is 

 often provided with a disc-like sucker, for holding on to surfaces ; the pharynx 

 also has a proboscis, for sucking or boring purposes. Of the parasitic Scole- 

 cida, the Nematoida, or thread-worms, have an alimentary canal, with both 

 inlet and outlet, a pharyngeal dilatation, and a simple intestinal tube, some- 

 times, however, dilated, so as to form a sort of stomach, and sometimes a 

 second dilatation lower down. In the Trematoda or fiukes, such as the dis- 

 toma, tristoma, and others, there exists either a double or a ramified canal, 

 with a common pharynx, but no anal aperture. In the Gordiacea, or hair- 

 worms, there is likewise no such outlet. The organization of all parasites, to 

 whatever class they belong, is more or less aberrant. 



In the Tseniada or tape-worms, and in the Acanthocephala, represented by 

 the echinorhynchus and echinococcus, also parasites living in the interior of 



