380 



Mr. W. H. Caldwell. On the 



[Dec. 7,. 



Trochosphsere, is from the earliest stages reduced relatively to the 

 post oral. 



This latter, appearing in the gastrula stage, persists throughout life 

 as a eircumcesophageal ring. No anterior dorsal sensory part of the 

 central nervous system exists in the adult. 



The ganglion of the praeoral lobe which in Chaetopoda and Mollusca,. 

 &c, persists as the anterior sensory lobe of the brain, disappears 

 with the change from a free to a fixed life. 



The ganglion sense organs and velum of the prseoral lobe are eaten 

 during the metamorphosis. 



The pair of sense organs are connected with the postoral nerve- 

 ring. 



In Capitellida?, Dr. Eisig has been kind enough to inform me, the 

 nerves from the ciliated pits are connected not with the anterior lobes 

 of the brain, but with the posterior part from which the eircumoeso- 

 phageal commissures are given off. 



Body-Cavity. 



The whole body-cavity in the prseoral lobe and in the trunk is an 

 enteroccel. The closed vascular system is developed from the spanch- 

 nopleure. The intracellular excretory canals arise in somatic mesoblast 

 cells. The existence of two divisions of the excretory system, viz., 



i. The intracellular closed canals ; 



ii. Large intercellular canals ; 



ceases with the metamorphosis. In Phoronis the atrophy of the intra- 

 cellular system is coincident with the development of the vascular 

 system. 



If the intracellular excretory system of larval Phoronis is homo- 

 logous with the similar excretory system in Platyelminthes, there- 

 is a presumption that the cavities in which the cells lie are homologous, 

 that in fact Platyelminthes are degenerate enterocceles. 



On the Relation of Phoronis to other Groups. 



The most striking result of my researches on Phoronis is to give an 

 explanation of the relation of Bracliiopoda and Polyzoa to other 

 animals. The identity of the Phoronis larva up to the formation of 

 the nephridia, and before the outgrowth of the anal region, with 

 the Trochosphsere type of Hatschek is complete. 



In Phoronis the body-cavity is an enteroccel. The distinction 

 attempted to be drawn by the Hertwigs* between the histological cha- 

 racters of mesenchyme and mesoderm utterly breaks down in Phoronis.. 



I regard it, therefore, as probable that the other Trochosphseres are 

 enterocceles. 



* O. and E. Hertwig, loc. cit. 



