VERMES. 



pus, fo a? to bring out the two liooks, which feize the food 

 l:ke a pair of forceps. Wlien it is feized, they drag it in, 

 and the mufcular part of the ocfophagus, acting on it by its 

 coiitraftions, and by means of the liorny papilla;, divides 

 and triturates it, and thus prepares it for the aftion of the 

 inteftinal canal. 



The other marine vermes, arranged near the nereids, 

 fuch as the arenicolse, the aniphinomice, amphitrite, lere- 

 bells, and ferpula?, have neither jaws nor teeth. At lead 

 we can hardly s^ive that name to the peftinated procefles of 

 the ampliitrite. They are fealy pointed pieces, of a 

 brilliant golden colour, arranged in two rows, which repre- 

 fent two combs, but fituated out of the mouth, on the fur- 

 face of tlie head, and enabling tlie animal to fix itfelf, or to 

 hook in various fnbllances, but not to mafticate or divide 

 the food. 



The aphrodita: have four fmall teeth at the bottom of a 

 probofcis, whicli they can extend or withdraw at will. 



Leeches have three fmall femi-circular prominences in the 

 interior of the mouth : the eda;e is cutting, and finely denti- 

 culated, like a faw. With this inftrument they pierce the 

 fkin. The lumbricu? has no jaws. 



Organs of Blajlicalion in the Ech'wo-iltrmala. — Amongfl. 

 the invertebral animals, the echini are thofe which have the 

 moft furprifing apparatus of this kind. Their external 

 covering, which is bony and confills of a fingle piece, prc- 

 fents a large round hole, in which the mafs of the mouth is 

 fufpended, attached indeed by ligaments and mufcles, but 

 moveable to a certain point. The bony part of this mafs 

 has feme refemblance to a lantern with fix divifions : the 

 comparifon was made as long ago as the time of Ariftotle. 

 The objetl of the apparatus is to fupport and move five 

 teeth, which encircle the fmall round aperture, by which the 

 food enters. Thefe teeth are worn away by maflication, and 

 are con(lru£ted on the fame principle as the incifors of the 

 rodentia ; viz. very long, loft behind, and hardening towards 

 the front, in which direction they advance in proportion to 

 the effedl of tlie attrition. They rell in an apparatus 

 confiding of fixed and moveable pieces. The fixed pieces ad- 

 here within the fliell, all round the hole : they confill of five 

 bony arches, whofe convexities are turned towards the cavity 

 of the fliell, or downwards ; while their concavities arc to- 

 wards the edge of the circular opening, or upwards. The 

 principal moveable pieces are five triangular jjyramids, form- 

 ing the principal body of the mafs of the mouth, and dividing 

 the great pyramid or pentagonal lantern of the mouth. Two 

 faces of cacii pyramid correfpond to thofe of the neighbour- 

 ing pyramids : they are marked by five tranfverfe Itrix. 

 Their inner edges do not touch each other, but are fe- 

 paratcd by a fmall interval. The dorfal or external face of 

 eacli pyramid is convex, thick, and perforated towards its 

 bafe by a triangular or circular opening, differing in fize ac- 

 cording to the fpecies. Its inner edge has a groove, in which 

 the body of the teeth pafTes and can move longitudinally, but 

 in no other dircftion. Its extremity paffes out at the point 

 of the pyramid ; and the five points being approximated 

 about the opening of the mouth, the five teeth end there 

 alfo. 



The pyramids are hollow, and their faces do not exactly 

 touch thofe of the neighbouring pyramids ; but they are 

 united by a flefliy mafs, which can approximate them. Its 

 effeft is that of bringing the five teeth together, and thus 

 contracting the opening of the mouth. 



The canal of the celophagus paffes between the five py- 

 ramids : the fides of their bafes, by which they touch each 

 other, are united, two by two, by five bony pieces difpofed 

 like radii, and approximating towards the ocfophagus as 

 Vol. XXXVII. 



their centre. Each of thefe pieces unites the adjacent fides 

 of the bafes of two pyramids, being articulated to them in 

 a loofe manner. The third fide of the bafis of each py- 

 ramid, that which conllitutes the bafis of its dorfal or exter- 

 nal furface, forms one of the planes of the general pyramid 

 or pentagon. " In the natural pofition thefe fides correfpond 

 to the intervals of the fixed bony arches, which confequeutly 

 anfwer to tlie angles of the pentagonal pyramid. 



Twenty mufcles aft from the fixed bony arches on this 

 pentagonal pyramid, and can either move it entirely, or move 

 on each other the five triangular pyramids which compofe 

 it. Ten of thefe mufcles pafs from the intervals of the 

 arches to the external bafes of the five pyramids. When 

 they aft all together, while at the fame time the mufcles 

 joining the pyramids together contraft, the whole mafs 

 of the mouth is carried forwards, or towards the ouifidc of 

 the body. If they aft feparately, tliey incline the mafs and 

 render its axis oblique, makiug the internal extremity of the 

 axis converge towards the fide of the mufcles wliich aft. 

 If one afts alone, while the particular mufcles joining its 

 pyramid to the two neighbouring ones are relaxed, it carries 

 the tooth of that pyramid further inwards than the others. 



Tiie ten ollur mufcles go from the convexities of the 

 arches like radii, to terminate at the points of the pyramids; 

 fo that each point receives the mufcles of the two neigh- 

 bouring arc'ies. As the arches projeft inwardly, thefe muf- 

 cles are inclined towards the outer furface of the (hell ; 

 confequeutly their effeft, when they aft together, is that of 

 making the mafs of the mouth pafs a little inwards. When 

 they aft feparately, while the mufcles uniting the pyramids 

 are contrafted, they incline the mafs of the mouth, by 

 making the external extremity of its axis converge towards 

 the fide of the mufcle which afts. When the mufcles join- 

 ing the pyramid to its neighbours are relaxed, the effeft of 

 the mufcles we are now defcribing is to draw back the tooth 

 correfponding to tiiat pyramid, and move it away from the 

 aperture of the nyjuth. Thus, in thefe three relations, the 

 mufcles coming from the arches are antagoniiU of thofe 

 which come from their intervals. 



If both fets aft together, they become common antago- 

 nills of thofe which join the pyramids, and their operation 

 will then be to feparate the latter from each other, and to 

 enlarge, not only the entrance of the mouth, but the whole 

 of the paffage left for the ccfophagus through the axis of 

 the great pcntagoua! pyramid. 



Befides the twenty-five mufcles, which aft immediately 

 on the pentagonal pyramid and its parts, there are ten 

 others, which adt on it tlu-ough the intervention of five 

 officula, which we muft now defcribe. They are flender, 

 and rather femi-circular or arched ; and are placed each on 

 tiie fame level with one of the five bony radii which have 

 been defcribed. 



One extremity of each arc is articulated to the internal 

 extremity of the correfponding radiated piece : the other 

 paffes above and on the outfide of its external extremity, 

 and is bifurcated like the letter Y. A pentagonal membrane 

 unites and (h-engthens their extremities towards the centre. 

 Each of the two branches of tlie Y receives a mufcle com- 

 ing from the middle of the nearefi. interval of the fixed bony 

 arches ; fo that each of the five intervals gives a muiclo to 

 the hvo nearefl Ys. 



The effeft of the mufcles, afting by fucli levers, in, in-. 

 dining the mafs of the mouth in every dircftion can be, 

 eafily conceived. 



Eacli tooth may be confidered as a long triangular prifin ; 

 of which the two pollerior faces make re-entrant angles.. 

 The part which comes out of the point of the pyramid is 



E very 



