VERMES. 



I. Infusoria. 

 II. Polypi. 



III. Radiaria. 

 IV. Vermes. 



V. Insecta. 

 VI. Arachnid A. 



VII. Crustacea. 



VIII. Annelida. 

 rX. Cirrhipeda. 



X. MOLLUSCA. 



/tnimals without Vertebra. 



Generation by fplitting of the body, or by fhoots ; body gelatinous, 

 tranfparent, homogeneous, contraftile, and microfcopic : no ra- 

 diated tentacula nor rotatory appendices ; no fpecial organ, not 

 even for digeftion. 

 ' Generate by fhoots ; body gelatinous, with great powers of rege- • 

 neration ; no internal organ, except an ahmentary cavity with a 

 fingle opening. Mouth at one end furrounded by radiated ten- 

 tacula, or by ciliated and rotatory organs. They compofe, for 

 the mod part, compound animals. , 



" Suboviparous : great powers of reproduftion ; no head, eyes, nor 

 articulated limbs ; the form of the body radiated ; mouth placed 



_ below. 



' Suboviparous ; body foft, and highly reproductive ; undergo no 

 metamorphofis ; no eyes, nor articulated limbs, nor radiated dif- 

 pofition of internal organs. 



Oviparous ; undergo metamorphofis ; poiTefs, in their perfeft " 

 ftate, eyes in their head ; fix articulated limbs ; trachea: extend- 

 ing over the whole body ; a fmgle fecundation in the courfe of 

 hfe. 



■ Oviparous ; undergo no metamorphofis, but poffefs always articu- 

 lated Umbs, and eyes in their head. Trachea; confined to certain 

 parts ; an attempt at circulation ; feveral fecundations in the 

 courfe of life. 



f Oviparous ; body and limbs articulated ; flcin cruftaceous ; eyes on 



< the head ; and generally four antennae ; refpire by branchia; ; a 



l_ longitudinal knotted medullary cord. 



f Oviparous ; body elongated and annulated ; no articulated limbs ; 



\ fcldom eyes ; refpire by branchiae ; knotted nervous cord. 



I Oviparous ; pofTefs a mantle and articulated arms, with horny flcin ; • 



[ no eyes ; refpire by branchiae ; knotted nervous cord. 



{Oviparous ; body foft, with its parts not articulated ; mantle va- 

 riable ; refpire by branchiae, varying in form and fituation ; no 

 fpinal marrow, nor knotted longitudinal cord, but nerves ending 

 in a brain. 



DeofTees, 



ifl. No nerves; no vef- 

 fels ; no internal and 

 fpecial organ, but for 

 digeftion. 



2d. No knotted medul- 

 lary cord; no circu- 

 lating vefTels ; fome in- 

 ternal organs Ijefides 

 thofe of digeflion. 



3d. Nerves ending in a 

 longitudinal, knotted, 

 medullary cord ; re- 

 fpiration by tracheae, 

 which convey air ; cir- 

 culation imperfect, or 



4th. Nerves ending in a 

 brain, or a knotted 

 medullary cord ; re- 

 fpiration by branchise ; 

 arteries zuid veins for 

 circulation. 



Slrudure and Formation of the hard Parts, which fupply 

 the Place of the Skeleton in the lower Orders. — The want of 

 an internal articulated flceleton is the moft flriking charafter 

 of the fecond great divifion of the animal kingdom, or the 

 invertebral animals. Infefts and cruflacea have a fpecies 

 of external fkeleton ; they poffefs hard parts, which are at 

 once inftruments of motion, and means of fupport and pro- 

 teAion for the included fofter organs. (See Insects, in 

 Anatomy. ) The fhells of the mollufca are to be regarded 

 rather as provifions for defence, as habitations of the foft 

 animals which they inclofe, than, like the flceleton of the 

 vertebral animals, or the hard external covering of cruflacea 

 and infefts, as inflruments of motion. 



Shells are compofed, like bones, of a calcareous matter, 

 intimately connefted with a gelatinous fubflance, from which 

 it jnay be feparated by means of acids. It is not difpofed 

 in laminae, or in fibres, but is diflributed uniformly through- 

 out the whole body of the fhell. 



It is only in fome fpecies that we find ftrata eafily fepa- 

 rated, and as it were agglutinated to each other, like the 

 leaves of paper in the formation of pafleboard. We know 

 from obfervation that thefe flrata do not all exifl in young 

 animals ; they have only the external, which are at the fame 

 time the fmalleft. In proportion as the animal increafes in 

 age, it forms a new ftratum on the internal furface of the 

 thell, which extends beyond the edges of all the preceding 

 ftrata; fo that each operation of this kind adds to the fize 



of the fhell in length, breadth, and thicknefs. Thefe are 

 certain fafts : to prove them, it is only neceffary to compare 

 fome fhells of the fame fpecies that have belonged to indi- 

 viduals of different ages ; the fewefl ftrata will always be 

 found in the fhells of the young. Mufcles, which may be 

 obferved when they are very young, and even before they 

 quit the body of the mother, have at that period one ftra- 

 tum only ; but the fhell is not therefore foft and gelatinous ; 

 it pofTeffes the fame firmnefs as the adult fhell, and its 

 greater fragility is merely owing to its thinnefs. 



It has been a queftion among phyfiologifts, whether thefe 

 fhells grow by developement or intuffufception, or by fimple 

 juxtapofition ? That is, whether the fhell, hke our bones, 

 contains nutritive vefTels capable of increafing, diminifhing 

 or varioufly modifying it ; or whether the gelatinous and 

 calcareous component elements of the fhell are fimply 

 depofited from the furface of the animal's body, and attached 

 to the pre-exifting mafs ? We conceive that the latter mode 

 of formation has been incontrovertibly eilabhfhed ; that the 

 fubftance of the fhell is inorganic, and confequently pofFeffes 

 no power in itfelf of incrcafe, diminution, or any vital 

 change. 



This point was flrft inveftigated by Reaumur, whofe re- 

 fearches are fo clear and fatisfaftory, that they have left 

 very little to be added by his fucceffors. They are detailed 

 in the Memoires de 1' Academic des Sciences for 1709, under 

 the title " De la Formation et de I'Accroiffement des Co- 

 1 1 quilles 



