VERMES. 



The medufie differ moft widely in the manoer by which 

 tlie aliment enters the ftomach. Some have a fingle mouth, 

 a large round opening- : others, inilead of a mouth, have 

 numerous branched tentacula, each perforated by a fmall 

 oper.ing. Each opening gives origin to a Imall canal, 

 which joins the neighbouring one, and fo on : in this way 

 four large trunks are formed, which end in the ftomach, and 

 convey to it the matters abforbed by the fmall apertures of 

 the tentacula. The number of the latter fometimes exceeds 

 eight hundred. 



It is from this ftructure, which is hitherto unique in the 

 animal kingdom, that Cuvier has ellablilhed the genus 

 rhizoftoma, from two Greek words ( ,1^2 and ^o/j.x) fignifving 

 root and mouth. The rhizoftoma, in faft,may be faid to derive 

 itsnourifhment from a kind of roots ; and in it, as well as in 

 all the medufas, the ftomach fupplies the place of a heart. 



The alimentary apparatus of the aftiniae confifts of a 

 fimple bag, with a circular opening, ferving both for mouth 

 and anus. The aperture is placed in the centre of the fu- 

 perior furface of the animal, and is furrounded by the ten- 

 tacula, which can feize the prey, and convey it immediately 

 to the mouth. The animal has the power of contrafting or 

 dilating this orifice. The alimentary fac is fufpended in the 

 general cavity of the animal by a kind of membranous at- 

 tachment. No inteftine nor any veflel is known to proceed 

 from this ftomach. See Memoire pour fervir a I'Hift. de 

 I'Afterie rouge, &c. pur Dr. Spix, Annales du Mufeum, 

 torn. xiii. pi. 33. 



" It is furprifing (fays Reaumur), that a foft animal like 

 this, not provided with claws, or any thing equivalent, (hould 

 be able to devour others apparently well defended by their 

 (hells, fuch as mufcles and other bivalves, and various fpecies 

 of univalves. It is however certain, that the aftinias live on 

 the flefh of ihefe animals, though, as they fwallow them 

 whole, and then contradt the entrance of the ftomach over 

 them, it is not eafy to find out how they extraft the animal 

 from its ftielly coverings. We can only fee that after a cer- 

 tain time they expel the empty ftiells by the fame orifice 

 through which they had fwallowed the wliole animal. I have 

 feen in this way the largeft mufcle-ftiells thrown out empty by 

 moderate-fized aftiniae : while in fomc cafes they are re- 

 jefted without the animal having been extrailed. In the 

 fame way I have feen them throw up entire buccina. I once 

 faw a large mufcle expelled entire through the bafis of the 

 aftinia, where there is no natural opening. In getting rid 

 of the fhelk, particularly when they are large, the animal 

 not only dilates its mouth to the greateft extent, but abfo- 

 lutely inverts the whole cavity, as you would a ftocking." 

 See fig. 25. Reaumur, Acad, des Sciences, 1 7 10. p. 475. 



In the common polypes (hydra), the whole body appears 

 to be a ftomach ; and the nutritive matter is imbibed appa- 

 rently direftly from the fm-face of the cavity into the fub- 

 ftancc of the animal. The moft curious fatt in relation to 

 this ftomach is, that if the animal be inverted, the external 

 furface performs the office of ftomach juft as well as the ori- 

 ginal ftomach did. 



The pyrofoma, a large fpecies ot marine polypus, without 

 arms, brought to France by Pcron, feems, like our frefli- 

 water polypes, to be a mere ftomach. 



The polypes, which form by their aggregation compound 

 animals, fuch as tliofe which produce the various lithophytes, 

 have a ruitritivc fyftem nearly related to tliofe of the common 

 polype and medufa. Cuvier has examined this in the vere- 

 tillx (pennatula cynomorium), whofe large and foft body, 

 and tranfparent polypes, arc more favourable to fuch re- 

 fearches than moft other animals of this clafs. In the body 

 of each polype, a fmall ftomach with brownifti parictes is 



obferved, from which proceed hve lubes funilar to thoit of 

 the medufie, that is, executing the funftions both of in- 

 teftines and veffels. Thefe inteftines are at firft yellowifti 

 and undulated ; having traverfed two-thirds of the lenrth 

 of the polype, they become ftraight and fmaller, and thus 

 penetrate the general body or ftem which fupporls all the 

 polypes. They then feparate to join correfponding veffels 

 from the neighbouring polypi, and form with them a net- 

 work occupying the whole mafs of the ftem. By means of 

 this communication, the food taken by one polype is en- 

 joyed by the whole animal, which may be confidered as a 

 fingle one with feveral mouths and ftomachs. 



The alcyonium exos exhibits an analogous ftrufkure. See 

 Dr. Spix, in the Annales du Mufeum, torn. xiii. p. 451, et 

 feq. pi. 33 ; and it is probable that a fimilar organization 

 prevails through the whole clafs. 



Appendages io the AHmerUary Canal. 



Liver. — All the moUufca have a liver, which is generally 

 very large, but never poflefTes a gall-bladder. It does not 

 receive, as in the vertebral animals, the blood which has cir- 

 culated through the inteftines, and tlius acquired a venous 

 nature ; but it derives from the aorta the neceffary fupply for 

 its own nutrition, and the fecretion of its peculiar liquor; and 

 it returns tiiis blood to the vena cava, which is the fame with 

 the pulmonary artery in thefe animals. In this arrangement 

 we may perhaps find a reafon for their having no fpleen. 



The liver of th^ cephalopoda is a large oval mafs of a 

 yellowifh-brown, fituated towards the back near the head, 

 partly filling the interval behind the funnel, and partly 

 defcending into the abdomen. It may be divided into two 

 lobes, between whicli the trunk of the aorta pafTes, giving 

 to each a confiderable branch. The bag, wiiich produces 

 the inky fluid peculiar to thefe animals, is inclofed between 

 thefe two lobes ; and in the calmar (fepia loligo), it is at- 

 tached in front of them. Monro confidered it to be a gall- 

 bladder ; he thought the ink was merely bile, confequently 

 that that fluid was excrementitious in thefe animals. This 

 is a grofs error. In the common cuttle-fifti the ink-bag is 

 found in the bottom of the abdomen, far from the liver ; 

 and in thofe fpecies, where the two organs are neareft to- 

 gether, they are not organically united. The bag contains 

 its fecrcting apparatus in its own cavity, and the liver 

 pours the bile into the alimentary canal. There arc two 

 excretory tubes, one for each lobe, penetrating together the 

 third ftomach, near its middle. Air impelled into the hepatic 

 vein pafTes eafily into thefe two canals ; and they fpeedity 

 inflate the third ftomach. The bile whicli they pour out 

 is of an orange-yellow : it remains for a confiderable time 

 mixed with the chyme, in the lateral and tortuous refervoir 

 of t!ic third ftomach, where it can flowly exert its aftion. 



All the gafteropoda have a large liver, divided into nu- 

 merous lobes and lobules, and lomctimes into feveral maffes, 

 each of which has a particular excretory canal. Thefe 

 iobes are interwoven with the inteftinal convolutions, en- 

 veloping them, or being enveloped by them, and united by 

 a cellular texture. The diftribution of the artery and vein 

 IS eafily feen, and even that of the proper vellels, which are 

 diUnbuted into the fmalleft lobules, the liver relembling a 

 bunch of grapes more than a homogeiieou!> parenchymatous 

 mals, and extending ufually through nearly the whole 

 length of the body. In the aplyfia, it pours out its fecre- 

 tion by feveral openings near the orifice of the cicum, cr 

 fourth ftomach ; that is, nearly as in the cephalopoda. In 

 the pleurobranchus and onchidiuni, which have fcvcr.il fto- 

 m.ichs, there are difterences. The bile is poured into llie 

 firft ftomach of the pleurobranchus. The onchidium has 



its 



