VERMES. 



may be conceived that the protraftors, when they aft, pufh 

 the retraftor outwards ; but the latter, when contrafting in 

 its turn, tends to recover the parallel fituation of its fibres, 

 and thus draws the fpines inwards. It is by the help of 

 thefe mufcles, and of the fpines on which they aft, that the 

 imperfeft locomotion of thefe worms is effefted. 



There are other worms, deftitute both of fpines and 

 bridles ; and therefore pofTeffing a different mufcular or- 

 ganization. Their manner of crawling differs confiderably 

 from that of the former. Their progrefllon is accomplifhed 

 by means of the two extremities of their bodies, which they 

 apply alternately to the furface on which they crawl. They 

 are fitted for this kind of motion by a peculiar ftrufture. 

 We mav di\nde them into two orders. 



The firft, as the leeches, and fcvcral inteftinal worms, have 

 the head and the tail terminated by a kind of contraftile flefhy 

 dific, fomewhat refembling thofe of the arms of the cuttle-fifh. 

 The ftrufture of thefe two difks, which perform the office 

 of fuckers, cannot be eafily afcertained ; for when the fkin 

 which covers them is removed, we obferve merely fome very 

 fmall fibres interwoven in different direftions. 



Though the worms with fuckers polTefs a great power of 

 contraftion, it is extremely difficult to trace the mufcles that 

 move their bodies. Their whole fl<in may indeed be regarded 

 as one mufcle, or kind of flefhy fac, furnifhed with circular 

 and longitudinal fibres, and contaip.ing the veffels, vifcera, and 

 glands. This mufcular fldn is thick, and li«ed with a very 

 lolid and compaft cellular fubftance. 



When the worm wifhes to change its place, the body is 

 fixed at one of the extremities, by means of the fucker that 

 terminates it ; the circular mufcles of the n<in then aft, 

 which elongates the animal's body by diminifhing its dia- 

 meter : when the free extremity has in this manner reached 

 the place to which the wonn choofes it fhould be extended, 

 it is applied and made fafl to that fpot by the fucker, and 

 becomes the fixed point of a new motion : the aiiimal 

 having detached the fucker firft made ufe of, draws it by 

 the operation of the longitudinal fibres of the flcin towards 

 the fecond fucker, aad proceeds in this manner to fix each 

 extremity alternately. This is the mechanifm by which 

 progrefllon is effefted in worms that have terminating dillcs. 



The fecond order of worms, which move by fixing their 

 extremities, includes the greater part of the inteftinal kind. 

 Thefe pofTefs lefs contraftile power than the leeches, and 

 their motions are therefore lefs extenfive. Their head, in- 

 ftead of being terminated by a dilk, is fometimes provided 

 with hooks, by means of which they fix themfelves to the 

 parts they fuck. Such are the common tinia, the tsnia fo- 

 lium, the hydatigena, the hsruca, the echinorhynchus, the 

 uncinaria, &c. &c. The difpofition and number of the 

 hooks, which vary confiderably, have been defcribed by 

 naturalifls. 



The Organs of Motion in Zoophytes vary confiderably in 

 their nature, form, and aftion. It is necetfary, therefore, 

 in order to obtain a juft notion of thefe organs, to take a 

 particular and fuccefCve view of them in certain orders of 

 thofe animals. 



The echino-dermata are diftinguifhed by numerous re- 

 traftile feet, and a covering more or lefs folid. TheTe feet 

 are a kind of fuckers, and have nearly the fame organiza- 

 tion in the three genera which conipofe this order. In their 

 form, they refemble a globular phial or ampulla : they are 

 filled with a fluid, and their parietes are formed of circular 

 fibres. The elongated or tubuljr portion of the ampulla is 

 the only part that appears externally, when the feet are ex- 

 tended. It is terminated by a kind of difk, which is con- 

 cave in the middle. The fpherical portion is fituated within 



the body. From this conllniftion of the foot, the me- 

 chanif;n of its aftion will be eafily underllood. Tiic liquor 

 contained in the ampulla becomes, by a change of place, the 

 caufe of motion : when the foot is drawn into the body, the 

 fpherical portion of the ampulla is greatly enlarged : when 

 the foot protrudes, the parietes of the ampulla contraft, and 

 impel the contained fluid into the tubular part, which con- 

 fequently increafes both in length and circumference. In 

 the retraftile motion of the foot, the tunic of the tube is 

 contrafted, and the liquor thereby forced back into the 

 body of the ampulla. The number of thefe feet vary 

 confiderably in the different genera and fpecies. 



The holothuris are covered with a thick coriaceous flcin, 

 which the animal can lengthen or fliorten at pleafure. Thefe 

 two motions are produced by longitudioal mufcular bands, 

 varying in length and breadth in different fpecies, and fmaller 

 tranfverfe bands extended over the whole internal furface of 

 the body. The animals included in this genus have their 

 feet difpofed in different manners, and in fome fpecies they 

 are even wanting. In others we find them either fpread ir- 

 regularly over the wliole body, fituated upon one fide only, 

 or placed in longitudinal rows. 



In the afterix, or fea-ftars, the covering of the body has 

 a clofe fibrous texture, the interftices of which are filjed 

 with grains of calcareous matter of various forms and di- 

 menfions. This kind of cruilaccous fkin is however fufcep- 

 tible of a certain motion, which, though fiow, is very re- 

 markable. The body of the animal is commonly divided 

 into five branches, to which the feet are attached. Thefe 

 lall are ranged in fev^ral files throughout the whole length 

 of the branches from the mouth. The branches are fome- 

 times furnifhed with fpines, their middle portion is frequently 

 entirely calcareous, but articulated at its origin, and move- 

 able upon the central part of the body. 



Reaumur counted 1520 legs in a ftar-filh ; yet their mo- 

 tion is extremely flow. Thefe legs can be extended or with- 

 drawn, or partly thruft out : when withdrawn, their extre- 

 mity is vifible. Mem, de I'Acad. des Sciences, 1710, 

 p. 487. 



The echini, or fea-eggs, are encrulted by a complete cal- 

 careous fhell, the furface of which is covered by tubercles 

 difpofed in a very regular manner. Moveable fpines of va- 

 rious (hapes and fizes are articulated to thefe tubercles. It 

 is very difficult to difcover the fibres by which the fpines are 

 moved at the will of the animal ; for in their joints we ob- 

 ferve only a folid Ugamentous fubftance, which cannot be 

 eafily cut. The feet are protruded through holes which 

 perforate the fhell with much regularity, and form uniform 

 parallel lines, called by naturalifls ambulacra. They are 

 very numerous, but produce, as in the afterias, only a very 

 flow motion. 



The medufx fvvim, by difplacing the water with alter- 

 nate motions, rendering their bodies now Jlat, now convex. 

 Reaumur has a figure of one ; Acad, des Sciences, 1 7 10, 

 p. 478, pi. II. 



" Although," fays Peron, " the medufs are compofed 

 of a homogeneous jelly, without any appearance of fibres, 

 they poffcfs a truly furprifing power of contraftion. Cort- 

 flantly adtive 011 the furface of the waters, we fee them 

 alternately contrafted and developed. When the animal 

 comes from below towards the furface, he flrikes from above 

 downwards, and thus raifes himfelf in confequence of the 

 refi fiance of the water to this motion of his umbeUa. In order 

 to change the direftion of his courfe, he is inclined, fo that 

 the umbella forms a more or lefs acute angle with the hori- 

 zon ; in this cafe the direftion of the flroke, and confe- 

 quently the relillance being oblique, he is urged forwards in 

 7 the 



