PHORONIS ARCHITECTA— BROOKS AND COWLES. 93 



posteriorly, these fibres become massed into definite muscle tracts, about halfway back from the 

 vestibule to the ventral insertion of collar-trunk mesentery (figs. 44r, 44(/). These two ventral 

 longitudinal tracts, which are bilaterally placed one on each side of the ventral median line, 

 become separated, in most cases at least, from the ventral body wall in the region of the posterior 

 pair of blood corpuscle masses and the latter become rather closely associated with them (fig. 4('i). 

 We could not discover that these fibres were in any way related to the nephridia as has been 

 described for some species. 



In the region of the insertion of the ventral tentacles the muscle fibres of the ventral tracts 

 become again applied to the ventral body wall, but definite tracts are no longer present. How- 

 ever, in the trunk region these fibres form a definite tract, which is confined to the ventral body 

 wall, and it does not disappear until the perianal ring is reached (figs. 44 g, Ii, i). 



Another tract of muscle fibres present in species B., which does not seem to be developed in 

 other Art)noti'och;i', judging from existing descriptions, is that found in the region of the bases 

 of the tentacles. From the dorsal muscle tracts, where the most dorsal and anterior pair of 

 tentacles arises, muscle fibres are given off, which follow the bases of the tentacles and which 

 form a well-developed ring of muscle fibres. In other words, there is a ring of muscle fibres 

 which follows the line of insertion of the mesentery between the collar anil trunk cavities 

 (fig. 44e). 



A tract of muscle fibres, which also seems to occur only in Species B., is one composed of 

 only a few fibres, which are found running around the edge of the hood on the internal wall of 

 the same (tigs. 44 '/, h). Where the edge of the hood passes into the wall of the collar cavity 

 these fibres are seen to run on to the internal surface of the lateral wall of the collar and to 

 mingle finally with the fibres of the dorsal tract. The direction these fibres take when they pass 

 on to the wall of the collar reminds one very much of the fibres which Masterman (15) figured as 

 nerve fibres. 



On the internal ventral surface of the hood in both species of Actinotrochae there is a system 

 of muscle fibres arranged concentrically. They run almost parallel with one another and with 

 the edge of the hood (figs. 47 and 44<'). 



Beside the tracts of muscle fibres which have been described there are, lining the walls of the 

 collar and trunk, circular muscle fibres lying between the longitudinal muscle fibres and the 

 ectoderm. These have been generally observed by previous workers as have also the muscular 

 covering of the ventral pouch and the muscle cells of the dorsal blood vessels. 



Body cavities, mesenteries, etc. — Much difference of opinion exists as to the origin and limits 

 of the body cavities in the Actinotrocha and also as to the value of these cavities in determining 

 the phylogenetic history of Phoronis. 



Roule (20) stands alone in considering the Actinotrocha to have but one body cavity, which 

 is lined by an epithelium formed from mesenchymatous cells. He absolutely denies the presence 

 of any mesenteries. 



Through the kindness of Mr. Longchamps we have been able to study the Actinotrocha of 

 Phoronis sabatieri, and have found that the mesentery between the collar and trunk is present, 

 although it is less highly developed than in other species. We are unable, with the material at 

 hand, to give any opinion as to the presence of a mesentery between the preoral lobe and collar 

 cavities. 



Caldwell (3) claims that there are but two body cavities, and that these are separated by a 

 mesentery (collar-trunk mesentery of Masterman). 



Longchamps (12) is inclined toward the view of Caldwell, while Ikeda (!») finds the mesentery 

 dividing the lobe and collar, which, however, he says, is incomplete. Both of these investigators 

 recognize the presence of the ventral mesentery. 



Menon (17) finds three body cavities (preoral, collar, and trunk), a ventral mesentery, and 

 indications of a dorsal mesentery in the trunk. 



Masterman (1(>) considers that the Actinotrocha have five body cavities — an unpaired lobe 

 cavity, a paired collar cavity, and a paired trunk cavity. This idea is based on his study of the 



