92 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Numerous fibres, which Masterman .speaks of as passing down on to the ventral collar wall, 

 are massed in the Actinotrocha Species B. into two definite thickenings which are seen in fig. 44c 

 and fig. 40. These thickenings of the nervous tissue gradually approach one another as we trace 

 the sections backward and come to run along the same line as do the two ventral muscle tracts 

 of the collar, but before the line of insertion of the ventral tentacles is reached these thicken- 

 ings are lost in the subepidermal nervous tissue. 



We have not been able to make out either in sections or in surface mounts any definite nervous 

 tract running from the collar nerve ring along the ventral region of the trunk, although, as 

 before said, there is a subepidermal network of nervous tissue throughout the wall. It 

 will be remembered, how 7 ever, that above we have described two longitudinal dorso-lateral tracts 

 of muscle fibres, and there are quite numerous longitudinal muscle fibres in the ventral wall of 

 the trunk. There is also a fairly well-developed layer of circular muscles, and these, together 

 with the longitudinal muscles, give the appearance in surface views of the longitudinal tracts 

 giving off branches. 



The nervous system of the Actinotrocha of Phoronis sdbatieri is less highly developed, 

 judging from the specimens we have examined, than that of either Species A. or Species B. The 

 ganglion, or, as Roule (20) calls it, the " plaque cephalique," contains ganglion cells like those we 

 have found in other Acti/notrochse, which shows that there is something more present than a 

 simple subepidermal nervous system, such as Roule has described, in the Actinotrocha of Phoro- 

 nis sabatit //'. 



Muscular system. There is no doubt but that there is some diversity in the arrangement of 

 muscle fibres in the different species of Actinotrochse. A study of the two species, A. and B., as 

 well as the description of different species by other investigators, convinces us of this. 



In the study of the muscular system the best results were with material fixed in Hem- 

 ming' s strong solution and stained with Haidenhain's iron hematoxylin. These solutions make 

 the muscle fibres stand out very distinctly, whereas material fixed and stained with other fluids 

 shows them so feebly that the muscle tracts might easily be overlooked. 



Ikeda (9) has described a pair of bundles of muscle fibres springing from "the hind lateral 

 corners of the ganglion and running divergently downward until they insert themselves in the 

 collar walls between the first and second tentacles." These muscles, to which he has given the 

 name of "retractors," are present in the Actinotrochse Species A. and Species B. (tigs. 34, 35, 4.">. 

 45a). 



The "retractors" that Ikeda figures in the trunk cavity of one of the Japanese Actino- 

 trochse were not found in either Actinotrocha Species A. or Species B. 



Another pair of bundles of muscle fibres is found in Species B. They spring from the 

 wall of the hood at the sides of the ganglion, traverse the cavity of the hood and become 

 inserted on its ventral wall directly under the ganglion (fig. 45^). 



Certain tracts of muscle fibres are very highly developed in Species B. Transverse sections 

 (stained with iron hematoxylin) through the wall of the hood in front of the ganglion show 

 black dots spread over the internal dorsal surface of the hood, and these seem to be embedded 

 in the mesodermal lining. These dots are the cut ends of muscle fibres, and as the sections are 

 followed posteriorly, these dots gradually become massed about halfway between the ganglion 

 and the sensory papilla and represent the sectioned ends of a pair of longitudinal muscle tracts 

 which are bilaterally placed on the right and left of the median dorsal line (fig. 44). These two 

 thick tracts of muscle fibres extend posteriorly in the dorso-lateral regions of the Actinotrocha 

 and do not disappear until the perianal ring is reached. They are very characteristic structures 

 in Species B. (figs. 44 to 44A), but we have not been able to make them out in Species A. 



These muscle bands, no doubt, serve to draw the anal end of the body of the Actinotrocha up 

 to the oral end during the metamorphosis. They are the most highly developed muscle tracts in 

 the body of the Actinotrocha and their course is almost identical with the course of the 'dorsal 

 nerve-." that .Masterman describes. 



Examination of cross sections of Species B. in the region of the vestibule shows the cut ends 

 of numerous muscle fibres which are spread over the ventral surface of the collar. Passing 



