ON THE BRITISH ANNELIDA. 165 



of animals. In treating therefore of tlie organization of these parasites, we 

 are compelled to consider them, not as a class of animals, established on any 

 common, exclusive or intelligible characters, but as inhabitants of a peculiar 

 district or country." In the progress of this Memoir it will be rendered in- 

 disputable, that if the ' denominations ' established by the learned Hunterian 

 Professor among the Entozoa be applicable and rightful on the ground of 

 anatomical structure, the same terms, on the same plea, will become avail- 

 able in the methodical distribution of the Annelida. Anatomical inquiries 

 afford no sanction to the use of these phrases : There is no solid annelid, 

 neither is there a solid Entozoon. — Why then perpetuate the employment of 

 terms productive only of false conceptions ? Difference of habitat neither 

 suggests nor requires a corresponding difference of organization : species of 

 the same genus, and nearly allied, are frequently found to maintain existence 

 under very diverse physical conditions. The inference is, therefore, even 

 d priori probable and just, which demurs to the separation of the Entozoa 

 from the Annelida on the ground of a difference in the outward circumstances 

 of existence ; nor is the propriety of such division yet clearly sanctioned by 

 the evidence drawn from anatomy. There is no single trait by which the 

 separation is so much justified as in that of the absence in the Entozoa of the 

 transverse annuli so characteristic of the true annelid. 



From these observations it must be evident, that the interposition, as has 

 been done by Dr. Carpenter*, of the Rotifera between the Entozoa and An- 

 nelida, can only be regarded as the putting asunder, on hypothetic principles, 

 those whom nature has intimately united in the bonds of consanguinity. 



Surveying the Worm- class in its affinities to those, superior to the Anneli- 

 dan standard in the scale, the eye encounters the highest forms of the arti- 

 culate type. In the structure of the Annelida and the Myriapoda, marks of a 

 community of plan may be readily discerned. The leech and the earth-worm 

 on one side, and lulus on the other, occupy the verge of the frontier-line 

 dividing these two articulate families. Neither the leech nor the earth-worm 

 is provided with any special organs for atmospheric respiration. The so- 

 called respiratory sacculi have nothing whatever to do with the respiratory 

 process ; and though from the date of the early Essays of Duges to the pre- 

 sent time, they have received the fondest marks of attention from every suc- 

 ceeding anatomist, as beautiful evidences of design, fitting these humble 

 worms for the double luxuries of an amphibious life ; to these parts, hence- 

 forth, must be assigned a far different function. 



It is only in this particular that the leech and the earth-worm, whose orga- 

 nization is singularly similar, stand distinguished from all other Annelida ; 

 that in them, more especially in the leech, the intestine is so uniformly ad- 

 herent to the integument as to preclude the existence of the chylo-aqueous 

 fluid. The intestine is joined to the integument by means of a thick spongy 

 layer, blended intimately with pigmentary epithelium, and composed of ca- 

 pillary blood-vessels. This is the true apparatus of respiration, and the me- 

 chanism of its function will be afterwards explained. The general proposi- 

 tion may here be enounced, that, in all Annelids, the true blood system and 

 that of the chylo-aqueous fluid are inversely proportional to each other. 

 The greater the amount of the latter, the less the complexity of the former, 

 and conversely. In the leech the peripheral circulation is densely complex, 

 and the chylo-aqueous fluid is superseded. In the earth-worm the latter 

 fluid is present in small amount, and the proper blood system is, in this pro- 

 portion, less elaborately developed. Between these two important systems of 

 nutritious fluids, there exists in the economy of the Annelida a wonderful 

 * See Principles of Physiology, 1851. 



