64 PRINCIPLES OF NATURAL ARRANGEMENT. 



spect, the resemblance of the Biyozoa is really much closer to 

 the Tunicata and other Acephalous Mollusks, all of which ob- 

 tain their food by the agency of ciliary currents, than it is to 

 Zoophytes ; and further, although most Bryozoa have the ten- 

 tacles circularly disposed around their mouths, yet in some (as 

 in Plumatella, Fig. 24) they are set upon two "lobes" or pro- 

 jections, one on either side of the mouth, so that the radial 

 symmetry by which all Polypes are characterized is entirely lost. 

 It is in the conformation of the digestive apparatus, and in the 

 presence*of a rudimentary nervous system distinctly Molluscous 

 in its type, that the relation of the Bryozoa to the Tunicata is 

 most strongly marked ; whilst the absence of all trace of a heart 

 and circulating system is a mark of relationship to Zoophytes. 

 Altogether this group may be considered as forming a complete 

 transition between the Molluscous and the Zoophytic series, but 

 to be rather on the Molluscous than on the Zoophytic side of that 

 boundary line, which, in any system of classification, must be 

 drawn between them. 



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, 

 however, 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 re- 

 semblance to them ; placing them nearer or more distant, accord- 

 ing to the degree in which the characters of resemblance or of 

 difference 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 complete 

 as the deficiency in our materials permits it to be. 



44. It is a question of the greatest importance in Classifica- 

 tion, what characters are to be relied on for distinguishing 



