PHORONIS ARCHITECTA— BROOKS AND OOWLES. 91 



From the posterior side of the ganglion two tracts of nerve fibres pass out and can be traced 

 backward some little distance, but they are soon lost to view, as Lkeda (9) has found to be the 

 case when studying methvlene-blue preparations. 



Sections through Actinotrocha Species B. bring out quite plainly certain nervous tracts which 

 appear as thickenings of the subepidermal nervous tissue and which correspond in a large part 

 to the principal nerves described by Masterman. 



Anterior to the ganglion a section through the hood shows the parallel nerves which run 

 from the anterior side of the ganglion to the anterior edge of the hood. The boundary of these 

 nerves, as shown in fig. 4-1, is a little too definite. The subepidermal nerve tissue, which forms 

 a thin layer below the ectoderm cells, is not shown in the series of sections to be described. 



Following the sections posteriorly we come to the ganglion, which in this specimen has 

 become invaginated. together with the overlying epidermis, so as to form a pit. A cross section 

 through this pit is shown in fig. 44/;. The cavity of the pit is lined by epidermis, while periph- 

 erally the wall of the pit consists of the ganglion cells and the nerve fibres of the ganglion (tigs. 

 38— 44/<). The nuclei of the ganglia are easily made out. but it is only after staining very deeply 

 with iron hematoxylin that the cytoplasm can be seen. The imagination in the region of the 

 ganglion is unusual and is brought about by the violent contraction of the hood when immersed in 

 the fixing fluid. This undoubtedly is the same condition that Masterman (15) has described and 

 the same structure that he has homologized to the '"neuropore" of the Chordata, or that he lias 

 compared to the tubular dorsal nervous system of the same type as that of Balanoglossus. (Q. J., 

 Vol. XL. page 2;»5, 296.) It should be mentioned, however, that Masterman (Ub) in his answer 

 to Roule has admitted the error of his rather hasty conclusion. 



Menon(lT) ha-- recently described a tubular nerve ganglion for a certain Actinotrocha, but 

 the structure i> probably due to fixation. 



Immediately posterior to the ganglion a cross section shows two thickenings of the sub- 

 epidermal nervous system. These thickenings are what Masterman has described as the 

 dorsal longitudinal nerves and they can be traced from the ganglion. They are almost exactly 

 between the dorsal muscle tract and the epidermis of the dorsal wall. A little farther back these 

 so-called nerves are not quite as distinct, but when the region of the first pair of tentacles is 

 reached they become more prominent again and diverge, passing down the lateral walls along 

 the bases of the tentacles (tig. 41). They meet in the ventral region and thus form a ring-like 

 thickening of the subepidermal nervous system, which is undoubtedly the same that Masterman 

 has described as the ■"collar nerve ring" (tig. 42). Ganglion cells are demonstrable in this nerve 

 ring by staining deeply with iron hematoxylin (tig. 39). As we shall show in the account of 

 I lie muscular svstem there is a ring of muscle fibre which follows the nerve ring". 



Masterman says that "fibers pass mid dorsally as a pair of tracts, giving off branches to 

 the body wall and terminating in a nervous ring just anterior to the perianal band." In his 

 figures of sections, however, the pair of tracts does not show back of the most dorsal pair of 

 tentacles. In Actinotrocha Species B. there are no definite tracts of nerve fibres running longi- 

 tudinally from the region where the collar nerve ring passes obliquely downward from the dorsal 

 surface of the collar. Nerve fibres are undoubtedly present all along the dorsal wall, but these 

 are not massed together in tracts and are simply the fibres of the ordinary subepidermal nervous 

 tissue. The nervous ring in front of the perianal band is not present in the Actinotrochse that we 

 have studied. 



Masterman (15) finds that part of the nerve ring around the edge of the hood passes up to 

 the nerve ganglion when it reaches the insertion of the hood, and that numerous fibres also appear 

 to pass on to the ventral surface of the collar region. Live Actinotrochse. (Species A. and Species 

 B.), when examined under the microscope, do not show a branch of the nerve ring of the lobe 

 passing upward to the nerve ganglion. Sections also fail to show this condition, which is very 

 necessary to Masterman's comparison of the nervous system of Balanoglossus and the Actino- 

 trocha. Fibres from the nerve ring do, however, pass on to the ventral surface of the collar 

 region. 



S9369 i-oj 10—11 T 



