1882.] Structure, Development, and Affinities of Phoronis. 381 



The larvas of Bracliiopoda and Polyzoa I regard as modified from 

 the Trochospheere by the earlier attainment of the relation of the 

 ventral surface which in Phoronis is only accomplished during the 

 metamorphosis. 



Phoronis and Bracliiopoda. 



The conception of the body plan of Bracliiopoda arising in this way 

 involves an entirely new view of the homologies of the body surfaces. 

 The following are the chief points which seem to me to determine 

 these : — 



1. The prseoral lobe of the larva of Phoronis, and the so-called 

 " segment " which bears the eye-spots in certain larval Brachiopoda* 

 persist in part at least as the epistome of the adult. 



2. There is a postoral nerve-ring in all the Brachiopoda I have 

 examined) situated as in Phoronis in the ectoderm. 



3. In both the body-cavity of the praeoral lobe is separated from 

 that of the rest of the body by a septum. 



4. The " segments " of Brachiopoda are represented in Phoronis by 

 the three divisions of the larva. 



1. Preeoral lobe as far back as the septum. 



2. The rest of the body to the anal ring. 



3. The invaginated foot. 



An evagination similar to that in Phoronis of the third " segment " 

 takes place in Brachiopoda when the larva fixes itself (Morse, Terebra- 

 tulina.) 



5. The tentacles arise from the line of the nerve-ring and are in the 

 form of a horse-shoe, the outer curve of which is ventral. In the 

 middle of the inner curve there is a break in the continuity of the 

 tentacles. This interval is in the median dorsal line. On either side 

 lies one of the youngest pair of tentacles. 



6. The rectum when present lies in the posterior of the three main 

 divisions of the body-cavity formed by the ventral and two lateral 

 mesenteries (vide diagram B). The nephridia open to the interior in 

 the posterior, to the exterior in the region of the anterior chambers 

 of the body-cavity. 



7. The same four divisions of the intestine are formed in both 

 Phoronis and Brachiopoda. 



Finally, I would point out that the so-called segments of Brachiopoda 

 are at right angles to the ordinary Ghcetopod segments. This is easily 

 seen to be the case by reference to the diagram, where AB represents 

 the axis perpendicular to which ordinary segmentation takes place, 

 CD that perpendicular to which Brachiopoda have been supposed to 

 be segmented. 



* Kowalewsky, loc. cit. 



