$96 



STOMACH AND INTESTINE. 



Many species of Distoma or Fluke may be 

 regarded as types of an arborescent or rami- 

 fied digestive tube. From a mouth which 

 is suctorial and sometimes visibly muscular 

 a canal passes backwards, to divide into two 

 large branches. These run along the margins 

 of the oval and flattened animal, giving off 

 other branches ; from which proceed a final 

 series of anastomosing twigs. 



y. In many creatures closely allied to the 

 preceding by conformation and habits, this 

 ramified canal is reduced to its primary bi- 

 furcations, the ends of which are sometimes 

 slightly dilated. Occasionally there is an 

 enlargement, which has the situation of a pha- 

 rynx ; and which, in a few instances, encloses 

 an apparatus perhaps masticatory. In the 

 genus Diplostomum and others, a distinct set of 

 vessels, which occupies the immediate neigh- 

 bourhood of the intestine, has been supposed 

 to represent a chyliferous or vascular system. 

 As regards these latter forms of digestive 

 apparatus, it may be conjectured, that the 

 ramification witnessed in the Tcenia is re- 

 ferrible, not so much to that mere vege- 

 tative repetition of similar structures which 

 affects the whole animal, as to a merging 

 of the digestive in the circulatory function. 

 In any case, the more simple form of tube 

 last mentioned appears rather akin to an 

 advance, than to a retreat, of development ; 

 while it sometimes visibly coincides with the 

 appearance of a new system of canals, con- 

 nected with the circulation of a proper nu- 

 trient fluid. 



In the Nematoid Entozoa, the alimentary 

 canal is generally a straight tube, which oc- 

 cupies the axis of the vermiform animal, and 

 opens at its extremities. In most genera 

 as in the Trichina, Tricocephalus, Ascaris, 

 Slrongylus, and others it widens posteri- 

 orly ; where it often experiences a further 

 dilatation, which only ceases near the anus. 

 Rarely, other indications of separation are 

 added: an oesophageal dilatation, as in the 

 Ascaris lumbricoides ; or an enlargement cor- 

 responding in position to a stomach, as in 

 the Linguatula and Filaria. 



Rudiments of the organs accessory to di- 

 gestion have also been detected. Blind tubes 

 opening into the canal near its mouth are 

 found in several genera : and the position of 

 these has sometimes led to their being re- 

 garded as salivary. While rarely there is a 

 tube which opens into the intestine in the 

 situation of a biliary organ. 



In the mode of attachment of their diges- 

 tive canal, this division of the Entozoa otfers a 

 marked contrast with the preceding. In the 

 Sterelmintha (or solid worms) the tube is 

 scarcely distinguishable from the mass of the 

 body. While in these Ccelclmintha (or hollow 

 Nematoid worms), it is suspended from the 

 wall of the belly by filamentous processes. And 

 though such an acquisition of an abdominal 

 cavity is no doubt partly referrible to the isola- 

 tion demanded by the organs of locomotion, 

 yet not only does this itself imply a genera! 

 'advance of development, but it is actually 



accompanied by a curious structure, which is 

 apparently connected with nutrition, and pos- 

 sibly renders the cavity of the abdomen the 

 receptacle of a kind of chyle. Its more perfe:t 

 form in the Ascaris lumbricoides may be briefly 

 described as consisting of a series of pyriform 

 processes, the peduncles of which are seated 

 immediately upon the mesenteric filaments 

 previously alluded to, and which project freely 

 into the abdominal cavity, so as to be sur- 

 rounded by the serum and grey transparent 

 substance that fills this space. Their shape 

 resembles that of the villi of higher animals; 

 and their size increases towards the median 

 line of the body. 



The alimentary canal of the Polyp exhibits 

 so wide a range of development, that while 

 by one extreme it approaches that of the sim- 

 plest anenterous Infusoria, by the other it 

 attains a complexity akin to that of the highest 

 Invertebrate. 



The Hydra is little more than a stomach or 

 sac, fixed by a sucker at its closed extre- 

 mity, and having at its other end a mouth sur- 

 rounded by prehensile tentacles. Digestion 

 is, however, energetic. The living prey, which 

 is paralysed by the deadly grasp of the ten- 

 tacles, undergoes a rapid solution in this sto- 

 mach, while its colours often visibly mix with 

 those of the parietes common to the organ 

 and theanimal; and finally, its excrementitious 

 residue is speedily rejected by the same orifice 

 through which it previously entered. 



In other solitary Polyps for example in the 

 marine Actinice the folded bag formed by 

 the stomach is separated from the mouth 

 by an oesophageal constriction. It is at the 

 same time attached to, and isolated from, the 

 general wall of the animal, by radiating mus- 

 cular bands ; which extend vertically down the 

 whole depth of the organ, so as to resemble 

 the septa of a poppy capsule as seen in a 

 transverse section. 



The compound Polyp appears chiefly to 

 vary from this type by virtue of its individuals 

 possessing a common stem, the proper nutri- 

 tion of which requires it to be closely con- 

 nected with the organ of digestion. Thus, 

 in some of the Antkozca which possess a 

 stomach very similar to that of the Sea-ane- 

 mones just described, an orifice of small size 

 at the bottom of the gastric sac seems to admit 

 the results of digestion into the general cavity 

 of the animal, within which they experience a 

 kind of circulation. 



In the Tubularian Polyp, the canal is modi- 

 fied by the addition of a structure which may 

 be regarded as a pharyngeal proboscis. It is 

 a globular projection, surrounded by tentacles 

 at its free extremity, and by its other end 

 received immediately within a circle of simi- 

 lar organs; the place of its attachment being 

 marked by an internal constriction, through 

 which the cavity of this appendix communi- 

 cates with that of the stomach. 



In many of these Polyps, the stomach 

 has been seen to possess a ciliated lining ; 

 and there are grounds for presuming the ex- 

 istence of a similar structure in several other 



