NOTES ON ACTINOTROCHA. 4/0 



tissues of the two surfaces are found small spaces separated 

 from one auotlier by bridges of basement tissue with a few 

 nuclei. The epithelium covering the oral surface consists of 

 columnar ciliated cells with long nuclei, while over the aboral 

 surface it consists of cubical cells ; along the free edge the 

 cells are longer and the nuclei more closely arranged. Tlie 

 collar coelom, which is now a definite space having a well- 

 marked coelomic epithelium, extends only into the base of the 

 epistome, and is separated from the peripheral spaces by a 

 mesentery. Further extension of the coelom into the epi- 

 stome must take place in later stages, as the cavity of the 

 epistome is stated to be crelomic. In considering the 

 homology of the epistome it must also be remembered that 

 the cavity of the preoral lobe of the larva, of which the 

 epistome is considered to be a remnant, is, whatever its nature 

 may be, quite distinct from the collar cavity, from which it is 

 separated by a well-marked mesentery. The occurrence, 

 though rare, of old Actinotrochas without anv trace of the 

 preoral lobe is also interesting in this connection. Thus 

 there can be no reason to doubt that the epistome is not a 

 remnant of the preoral lobe, but is a new structure, developed 

 as an outgrowth of the collar region. 



The most important points adduced by Masterman, in 

 establishing the Chordate affinities of Phoronis, have refer- 

 ence to the following structures in Actinotrocha : — (1) five 

 body-cavities ; (2) proboscis pores, collar pores, and trunk 

 uephridia ; (o) two notochords ; (4) subneural gland (hypo- 

 physis), which is homologised with the proboscis vesicle of 

 Balanoglossus ; and (5) the tubular nerve-ganglion. These 

 structures will be dealt with in the order given. 



(1) Body-cavities. — There are three principal cavities in 

 Actinotrocha. It will be convenient to adopt Masterman's 

 names, and to call them the cavities of the preoral lobe, 

 collar, and trunk. These spaces are separated from one 

 another by mesenteries. The mesentery between the preoral 

 cavity and the collar cavity can be roughly compared to a 

 Avatch-glass with its edge attached to the body-wall, and with 



