ON THE PRINCIPAL OF CEPHALIZATION. 171 



1 by arrow O) follow the same direction, that is, from 

 posterior to anterior, ventrally. Thus in Tumicata the 

 two openings are posterior and posterior dorsal ; the pos- 

 terior dorsal, being the anal or excurrent orifice; this is 

 always the shortest in Tunicata. In Lamellibranchiata 

 the anal tube moves nearer the branchial tube ; in the lower 

 forms their outer covering coalescing and of equal length, 

 while, higher up, the tubes becoming entirely separate, and 

 in some of extreme length, the anal tube being the long- 

 est. In Pisidium and other forms the branchial tube dis- 

 appears, and water is received through a ventral opening; 

 while the anal tube yet remains, occupying a posterior 

 position on a line with the antero-posterior axis, in the 

 same position the branchial tube occupied in the Tuni- 

 cata : and, finally, both tubes become nearly obsolete, and 

 the mantle is cleft all round, except dorsally. Thus the 

 progress of sac opening follows in the same line of rotation 

 with the intestine. The progressive regions of attachment 

 move in an opposite direction (Fig. 1, arrow A). Com- 

 mencing with the Polyzoa as the lowest class, we have, as 

 in the Cheilostomata, the dorsal portion large and spread- 

 ing, this being the fixed portion ; the anal opening being 

 turned toward this region, as in the Brachiopoda and 

 Tunicata. (The movable part of the ventral surface, 

 which is uppermost, being represented by the little lid). 

 This mode of attachment is the lowest feature; namely, 

 attached along the entire dorsal region. 



As we ascend to the higher forms of the class, we have 

 a freeing of the posterior portion of sac, and the viscera 

 permanently occupies this freed portion. In the Brach- 

 iopoda we have the sac free, held only by the peduncle ; 

 the means of- attachment springing anterior, and from the 

 dorsal valve, as in the partially freed Polyzoon. (Crania 

 and Descina are attached as in Lepralia). 



In Lingula, where we have the lengthened and flattened 

 sac, the animal stands vertical in the sand. In Terebratu- 

 la and allied genera, the dorsal valve already assumes 

 preponderance over the ventral valve, and now obtains its 

 normal position uppermost. 



All the Tunicates with few exceptions are attached, and 

 by their anterior end. 



