STOMACH AND INTESTINE. 



297 



species. Some of the circulatory movements 

 observed in their ingesta are perhaps accom- 

 plished by the aid of such an apparatus. While 

 their vigorous and almost peristaltic character 

 in other instances is clue to structures, the 

 voluntary and powerful contractions of which 

 entitle them to rank as muscles. 



The cilio-brachiatc Polyp possesses an ab- 

 dominal cavity occupied by fluid, in which the 

 alimentary canal is free to move. The canal 

 itself has a mouth and anus, which are both 

 situated at the free extremity of the animal ; 

 the former orifice being within, the latter 

 without, its whorl of tentacles. The mouth 

 opens into a pharyngeal dilatation, from which 

 a narrow tube leads into an organ analogous to 

 a gizzard. This organ possesses radiating 

 muscular fibres, and rliomboidal teeth, that are 

 capable of crushing its contents. Immediately 

 beneath it is the stomach, in shape like a two- 

 necked flask, and having its blind extremity 

 fixed to the attached base of the animal by 

 a retractory muscle. The pyloric aperture 

 is guarded by cilia, which rotate, and thus 

 delay, the food. The intestine is narrow and 

 simple, and its excrementitious contents are 

 expelled from the anus, to be immediately 

 hurried away by the current arising from the 

 action of the neighbouring cilia. 



The body of the Acalcphce generally con- 

 stitutes a disc with a fringed margin. It is 

 convex above, and concave below, with large 

 dependent processes. And it swims by what 

 seems to be an alternate preponderance 

 of contraction in these two surfaces. 



The condition of the alimentary canal is 

 here very remarkable. The Entozoa have 

 already offered us a ramified tube, that 

 could scarcely be regarded as strictly diges- 

 tive. But these Sea-nettles further com- 

 plicate this branched state by the posses- 

 sion of a central cavity. This is sometimes 

 placed between a convergent and a divergent 

 set of anastomosing canals ; and sometimes 

 approaches the stomach of the Distoma in 

 possessing the latter set only. In the latter 

 case, the so-called stomach communicates, 

 by a short and .simple tube, with the centre 

 of the lower or concave surface. And in 

 one species it also radiates unbranched tubes 

 which open on the margin of the disc. 

 The movements of the contents of these 

 canals seem to be effected by cilia. The 

 ramifications of the canals chiefly occupy the 

 under surface of the animal. 



The lartre order of Echinodermata again 



O D 



presents us with an important advance of 

 development in passing from its lowest to 

 its highest members. 



Thus the alimentary canal of the Aderias 

 has a single aperture on the under surface 

 of the animal. This leads by a short tube to 

 a central cavity, which divides into two pro- 

 cesses for each ray. These processes give off 

 secondary branches at right angles to them- 

 selves, and the latter end in tertiary cgeca. In 

 Comatiila the coeca disappear, and the canal 

 acquires a distinct month and anus, which open 

 near each other. In all, the canal is muscular, 

 is enclosed in a ciliated peritoneum, and has 



its primary divisions attached by a kind of 

 mesentery. 



In the Echinus, the anus generally opens on 

 the upper or opposite surface of the body. 

 Many of this genus have a complex masti- 

 catory organ, which is acted upon by powerful 

 muscles. The first part of the canal opens 

 into an intestine of much larger diameter, 

 opposite to a blind dilatation very like the 

 human caecum. The intestine is coiled twice 

 around the inner surface of the shell ; the 

 second coil reversing the direction taken by 

 the first, and both exhibiting a sinuous course. 

 Its width tapers awav to the anus. Its struc- 

 ture is delicate and transparent; it possesses a 

 mucous membrane, and longitudinal and trans- 

 verse fibres ; and it exhibits an intestinal vein, 

 which is especially marked towards the termi- 

 nation of the canal. 



In the vermiform Holothunce the canal 

 forms a kind of Z in the abdominal cavity; 

 passing first backwards, then forwards, and 

 again backwards to its posterior extremity. 

 The first part is wider and stronger than trie 

 rest, and its more glandular mucous membrane 

 presents longitudinal folds which terminate 

 in a slight circular one. Such a structure 

 causes this dilatation to be regarded as a 

 stomach. The narrowing intestine often ter- 

 minates in a large oval cloaca, into which 

 open two branching caeca.* 



The Annelida form a class of animals so 

 diverse in nature and structure, that it is diffi- 

 cult to include all the varieties of their diges- 

 tive apparatus within a mere brief sketch. The 

 canal always possesses a distinct mouth and 

 anus, which occupy the opposite ends of the 

 more or less elongated and cylindrical body. 

 Prehension is generally aided by teeth, which, 

 as in the Leech, perforate the skin of their 

 prey ; while in others as in some of the 

 Errantes it is effected solely by a proboscis. 

 In many of the marine Errantes the intes- 

 tinal canal is simple. In the Lumbrici the 

 canal soon dilates into a membranous pouch, 

 which is followed by a thicker and more 

 muscular portion, supposed to be a gizzard, 

 in some genera, this part of the tube is com- 

 plicated by being produced into pouches. 

 These are either numerous, as in the Leech; 

 or few, as in some kindred genera. Fi- 

 nally, in the Earth-worm, they are reduced 

 to mere constrictions of the canal ; while in 

 the Aphrodita, they are developed into tubes, 

 which expand, divide, and terminate as al- 

 most globular pouches. Clusters of glandular 

 follicles, which are supposed to be biliary, 

 open into the posterior half of the complicated 

 canal of the leech just alluded to : and analo- 

 gous structures are found in other genera. 

 In the Earth-worm, there is a singular ap- 

 paratus, the ti/plilosolc. This is a blind tube, 

 which occupies almost the whole length of the 

 canal, being attached to its dorsal aspect, and 



* Such a complex organization is strangely con- 

 trasted with the alleged fact, that the animal, when 

 alarmed, can shed the whole canal. This extraor- 

 dinary act is presumed to be voluntary, and is only 

 paralleled by the growth of another digestive ap- 

 paratus, which replaces that evacuated. 



