64 PRINCIPLES OP NATURAL ARRANGEMENT. 



The Tunicata possess a distinct heart, and system of blood- 

 vessels, no trace of which can be discovered in the Bryozoa ; and 

 this is a character of great importance. Moreover, they have a 

 nervous system, which is arranged upon the type of that of the 

 Mollusca, though in its most degraded form (ANIM. PHYSIOL. 

 435) ; whilst that of the Bryozoa, if discoverable, would 

 almost certainly present the radiated arrangement, as in other 

 Polypes. Besides, the power of propagation by budding is not 

 confined to the Polypes ; for it exists also in the lower Articu- 

 lata (ANIM. PHYSIOL. 729) ; so that it is not surprising to 

 meet with it in the lowest Mollusca also. Hence we shall pro- 

 bably be most correct, in regarding the Tunicata as Mollusca, 

 which exhibit links of transition to the Radiated sub-kingdom, 

 through their affinity with the Polypes ; whilst the Bryozoa 

 are Polypes which present strong points of resemblance to the 

 Mollusca, in the elevation of their plan of structure above that 

 of others of their class. 



43. It can only be, then, by the careful comparison of the dif- 

 ferent forms of animal structure, that we can select those, which 

 are entitled to rank as types of groups ; and it is in this that the skill 

 of the Zoologist is most displayed. Having satisfied himself, how- 

 ever, which are to be regarded as types i. e. as presenting their 

 respective plans of structure with the most completeness, he 

 proceeds to range around these such forms as have a close resem- 

 blance to them ; placing them nearer or more distant, according 

 to the degree in which the characters of resemblance or of differ- 

 ence predominate ; and taking care that the intermediate forms 

 shall be so arranged, as to form links of transition towards the 

 other types, of whose characters they partake. Whenever this 

 process shall have been accomplished for the whole Animal 

 kingdom (as it has been in some degree for certain divisions of 

 it), we shall be in possession of a Natural System, as com- 

 plete as the deficiency in our materials permits it to be. At 

 present, any general classification can afford but an approximation 

 to it ; and the one which is adopted in the following Treatise, 

 is not put forward by the Author, as by any means destitute 



