378 



Mr. W. H. Caldwell. On the 



[Dec. 7 r 



nervous system, for my fuller paper. The bearing of the facts of 

 Phoronis development on the question would involve matter of a too 

 speculative character. Suffice it to say, that if the nerve-ring of 

 Phoronis represents the nerve-ring of Ccelenterate ancestors, the 

 praeoral lobe must be regarded as a development of an anterior region 

 of the sub-umbrella, while the anus has been formed in the region of 

 the umbrella. 



The nervous elements of the ectoderm of the preeoral lobe in. all 

 species, are concentrated into a ganglion (Scheitelplatte). In some 

 species a large number of nerve fibres pass forwards from it to a sense 

 organ. In one species four eye-spots are present. Behind the mouth 

 an even number of tentacles form a postoral circlet. Behind these, 

 and corresponding in number, lie rudiments of the adult tentacles. 

 Along a line immediately in front of the larval circle, the ectoderm 

 cells have become vacuolated. This change extends to a breadth of 

 only three or four cells. Along a line at the base of the rudiments of 

 the adult tentacles, the nervous prolongations of the ectoderm have 

 formed a definite ring. 



Round the anus a ring of very columnar ectoderm bearing strong 

 cilia forms the chief organ of larval locomotion. The mouth opens 

 into an oesophagus, which leads into a stomach. The stomach at its 

 anterior end is produced into one or two ventral recesses. 



In the vacuolated walls of these structures brown concretions are 

 present. 



The septum is attached in a circle along the line of the nerve-ring, 

 and free communication exists between the body-cavity in front of 

 the septum and the split in the splanchnopleure, which will form the 

 blood sinus a.nd vessels of the adult. 



The condition of the rest of the vascular system we have already 

 described in an old larva. The muscular arrangement in the invagi- 

 nated foot is already similar to that in the adult. 



The ventral mesentery still exists along the whole ventral surface 

 from septum to anus. The pair of nephridia lie on either side of the 

 body, their numerous excretory cells floating freely in the body-cavity 

 in front of the septum. The external openings are placed one on 

 either side of the opening of the foot. 



The animal now swims to the bottom, and after swimming round 

 and round many times on its own axis, and meanwhile undergoing 

 violent contractions, suddenly begins to evaginate the foot. In 

 fifteen to twenty minutes, a healthy individual will have become in 

 all essential points like the adult. 



During this time the following events take place : — 



i. The whole praeoral lobe with ganglion and sense organs pass 

 into the stomach by the oesophagus. The rupture takes place along 

 the line of vacuolated ectoderm mentioned above. 



