532 HISTORY OF SYSTEMS. 



animals ; and that the former are naturally wide apart from 

 the latter, Lamarck thinks is proved by the following con- 

 siderations : — 1. Because the condition of existence of the 

 Tunicata, their generally attached state, their internal struc- 

 ture, and their peculiar outward form, are all alien to what 

 we observe in the Mollusca, none of whom have their organs 

 exactly paired and symmetrical. 2. Because their position 

 amongst the Mollusca rests upon the attribution of functions 

 to organs whose real nature and office are only hypothetically 

 determined. 3. Because the relative position and structure 

 of the branchial and alimentary cavities are dissimilar to 

 those of the Mollusca. The supposed branchial sac of the 

 Tunicata, for example, has an external aperture which ad- 

 mits the food into it, while what is said to be the real mouth 

 is placed at the bottom of this same cavity, — a disposition 

 of parts unexampled amongst moUuscans, even of the lowest 

 testaceous acephales, in which the branchiae are very dif- 

 ferently constructed. 4. Because Nature doth not seem 

 ever to have placed the branchiae in the alimentary canal 

 itself; and hence a trellis-work of nervures crossing each 

 other at right angles to form quadi-angular meshes is pro- 

 bably not vascular, but rather the result of muscular fibres 

 fitted by this disposition to contract and dilate the so-called 

 branchial sac : all blood-vessels depart from a straight course 

 only by making a curvature. 5. Because true branchiae are 

 only to be observed in animals which have a circulation, — 

 the existence of which in the Tunicata is not proved ; and 

 to admit the existence of a circulation in the Botryllidae 

 and Pyrosoma would seem to be ridiculous. 6. Because 

 the existence of a brain, of a heart, of a liver, and of genera- 

 tive organs, all essential to a molluscan, cannot be proven in 

 the Tvmicata, where, in these respects, all is conjecture and 

 arbitrary determinations. — Some of Lamarck's statements in 

 this argument have been disproved, and the others have 

 been deemed invalid by so many naturalists that we have 

 all along treated the Tunicata as members of the molluscous 

 class, and hence the reason that I introduce Lamarck's 

 classification of the latter with his arrangement of the 

 Tunicata.* 



Class TUNICATA. (August, 1816.) 



Order L Tuniciers reunis ou Botryllaires. Animals 

 agglomerated, always united together, constituting a com- 

 mon mass by their union, and apparently having a commu- 

 nication with each other. 



* See note at the conclusion of this letter. 



