ON THE PRINCIPLE OF CEPHALIZATION. 165 



ered, and tested them, by personal examinations of the an- 

 imals, it was not till I comprehended the importance of 

 the sac character, and understood the "Principles of Ceph- 

 alization" first enunciated by Prof. Dana, that I was enabled 

 to clear up previous doubts, discover new relations, and, as 

 I believe, rightly interpret the relations of the classes. 



"As the principle of Cephalization is involved in the very 

 foundation of the diverse forms that make up the ani- 

 mal kingdom, we may look to it for authoritative guid- 

 ance, with reference to the system that prevails among 

 these forms."* 



In the following considerations, all preconceived ideas 

 regarding the relative positions of the dorso-ventral, and 

 antero-posterior diameters of the animal must be laid aside, 

 and the essential structure of the animal if rightly under- 

 stood, must be our guide. The gradual morphological 

 changes of the contents of the sac, and all other relations, 

 are based on the principle of Cephalization. In the plate 

 presented (Series I) I have given a typical figure of the 

 six prominent groups of the Saccata ; namely, Polyzoa, 

 Brachiopoda, Tunicata, Lamellibranehiata, Gasteropoda, 

 and Cephalopoda. 



For obvious reasons, only the intestine, head, and pedal 

 ganglia within the sac are represented. These six figures are 

 placed in their normal position, anterior pole downward, 

 the dorsal region is turned to the left. Commencing with 

 the Polyzoa (Series I, P) we have the sac closed, while 

 the mouth and anus terminate close together at the pos- 

 terior pole of the sac; the mouth occupying the extreme 

 posterior position, and by a dorsal bend of the intestine 

 upon itself, terminating dorsally. The nerve mass is found 

 between the oral and anal openings. In this class the 

 mouth and anus have the power of protrusion from the 

 sac. In the three lower orders, Cyclostomata, Ctenos- 

 tomata, and Cheilostomata, the polyzoon, when complete- 

 ly evaginated, presents no fold or inversion of the sac, 

 while in the higher group PhylactolaBmata, there is a partial 

 and permanent inversion of the sac under like condi- 

 tions. 



* "Classification of animals based on the principle ; of Ceplialization." Dana, 

 Amer. Jour. Sci., Second Series, Vol. XXXVI., p. 321. 



