1882.] Structure, Development, and Affinities of Phoronis. 379 



ii. The larval tentacles follow the prgeoral lobe. 



iii. The blood-corpuscles pass inside the splanchnopleure sac by the 

 opening described above, and break np in the siniis. From this they 

 pass by contractions of the sac into the caecal vessels and into the 

 vessel which already exists in the dorsal side of the stomach. 



iv. The larval excretory cells of the nephridia break off from the 

 large canal and float freely into the body-cavity in front of the septum. 

 They pass with the blood-corpuscles into the vessels. The large 

 canals remain as the paired nephridia of the adult. The external 

 openings by the changes undergone during the evagination have 

 already almost their adult position. 



v. The body-wall of the anal cone (at this stage) becomes folded, so 

 as to present the appearance of columnar epithelium. By this process 

 the invagination of the whole anal cone is rendered possible. The 

 original anus is now half-way up the course of the ascending limit of 

 the alimentary tract inside the foot, and the adult position of the 

 nerve-ring is thus brought about. 



The ventral mesentery extends along the outer curve of the alimen- 

 tary tract along its whole extent, attaching the foot to the body- wall. 

 At the end of the body it is seen in end view, apparently as a linear 

 band, presenting a similar appearance to the funiculus of a polyzoon. 



The ectoderm, from what has been said, must now re-attach itself by 

 & secondary growth to the endoderm, along the lines where the 

 praeoral lobe broke off. 



In the stomach the disintegrated cells of the tentacles and the 

 prseoral lobe with its ganglion and sense organs are now digested, 

 not, however, by intercellular method, but, as in the adult, in the 

 •canal itself. 



General Conclusions. 



The life history of Phoronis, the chief points in which have been 

 hriefly mentioned, seems to offer a solution of many morphological 

 problems. 



These are of two kinds. 



On the one hand we have those more general questions which con- 

 cern the origin of various organs and systems of organs. 



On the other those special problems which are concerned in solving 

 .the body plan of the different animal forms. 



On the Origin of Organs and Organ Systems. 



The condition of nervous concentrations in the ectoderm in Phoronis, 

 both larval and adult, shows us how apparently new parts of the 

 .nervous system arise. 



In Phoronis the preeoral ring, corresponding to the velum of a 



