ON THE PRINCIPLE OF CEPHALIZATION. 177 



Polyzoa and Cephalopoda, we place in a horizontal 

 position, taking a swimming Dibranchiate for comparison : 

 this may be premature however. 



Brachiopods and Gasteropods with posterior pole slight- 

 ly elevated, as in Cyrtia and allied forms of Brachiopods, 

 and any coiled Gasteropod for example. Tunicates and 

 Lamellibranchiates with the axis vertical, the anterior pole 

 being below, and the Monomyarian horizontal again. It 

 must be remembered that the above considerations are 

 taken in their most general sense, representing only the 

 mean for each group, many of them perhaps erroneous. 

 They are given rather for the purpose of indicating a 

 future path of inquiry, which the writer considers fruitful 

 and intends to follow, than as points in any way settled. 



In ascertaining the mean position of the antero-posterior 

 axis for the whole branch of Saccata, (that is, the average) 

 we find that a line at an angle of 45° would represent its 

 position in nature ; the lower end being anterior. In the 

 Radiates a line through the mouth to the opposite region 

 of the body would stand vertical. In Articulates the an- 

 tero-posterior axis would be horizontal. Among the Ver- 

 tebrates, Fishes would be horizontal, as in Articulates ; 

 Reptiles have the head slightly elevated ; Birds and Mam- 

 mals still more elevated ; so that a mean line, for these 

 classes might be drawn at an angle of 45,° the cephalic 

 region being uppermost. Man stands vertical. Thus in a 

 diagram we would have the following : 



Man. 



Vertebrata. 

 Articulata. 

 Saccata. 

 Radiata. 





Fig. 2. 

 In the preceding considerations I have endeavored to 

 show the importance of the sac, as the principal and promi- 

 nent feature in their plan of structure. All animals, re- 



ESSEX INST. PROCEED. VOL. IV. W. 



