372 



Mr. W. H. Caldwell. On the 



[Dec. 7, 



Unfortunately, I have been unable to confirm Dr. Hatschek's 

 account in several points. To do justice to this, I reserve a fuller 

 account of the controverted points for my complete paper, when I 

 hope also to reproduce Dr. Hatschek's figures of the living larva3.* 



Structure of Adult Phoronis. 



The mouth and anus are situated at one end. The short line 

 between them is the median dorsal line. Between mouth and anus 

 lies an epistome. This epistome is the persistent prceoral lobe of the 

 larva. New tentacles of the lophophore round the mouth are added 

 on either side of the median dorsal line. There is a mesoblastic 

 skeleton in the lophophore. The ventral surface is produced into a 

 " foot," which constitutes the main part of the animal. 



This determination of the surfaces depends on the development to 

 be described below. 



Epithelium of the Body. 



Nervous processes of the ectoderm cells retain their connexion with 

 the ectoderm, and concentrations, both of fibres and ganglion cells, 

 occur in the skin outside the homogeneous basement membrane. 

 The central nervous system remains therefore in the epidermis, 

 representing the primitive condition. 



Concentrations of the nervous system take place round the mouth 

 to form a postoral nerve-ring, The anus lies outside this. The ring 

 follows the line along the base of the tentacles, and has therefore like 

 them the form of a horse-shoe. In front of this ring are situated a 

 pair of sense organs, which I shall speak of as " ciliated pits." They 

 lie in the concavity of the lophophore, on either side of the anus. 

 They have the characteristic structure of sensory epithelium, con- 

 sisting of sense cell, ganglion, and nerve-fibres. Sarsf has figured 

 in Rhabdopleura a pair of ciliated protuberances in what I hope to 

 show is an homologous position. 



A further concentration takes place in the form of a cord, which 

 runs from the median dorsal part of the nerve-ring two-thirds of the 

 length of the foot along its left side. It is therefore asymmetrical, 

 and lies in the epidermis outside the basement membrane. Inside 

 this nerve-cord lies an apparently hollow tube. This tube recalls 

 the so-called large fibres of Cheetopoda. 



The alimentary canal is in the form of a ciliated tube, which may 



* It is not to be understood that Dr. Hatschek agrees with my account of the 

 facts of development. 



f Gr. 0. Sars, " On Kliabchpleura mirabilis," " Q. J. M. S.," toI. xir, new 

 geries. 



