ii8 



04, p. 97) of a general nerve-plexus in the epidermis of the entire lophophoral region. Schultz 

 03, I, p. 406) finds that in regenerating specimens of Phoronis the central nervous system 

 develops on the oral side of a dorsal invagination, vvhich he compares with the medullary plate 

 of Yertebrates and with the dorsal centre found In the collar of Balanoglossus. In his second 

 paper (03, 2, p. 477) he confirms this homology, with the modification that he regards the 

 invao-inated ganglion as an adult organ which replaces the original larval ganglion, stating that 

 a process takes place in Actinotrocha, at the time of metamorphosis, identical with what he 

 describes in the regenerating Phoronis. 



I formerly called attention (87, p. 46) to the resemblance between the ovarian mesenteries 

 of Cephalodiscus and the lateral mesenteries of Phoronis^ although suggesting that it was by 

 no means certain whether the oviducts could be regarded as the representatives of the nephridia 

 of PJwronis. Masterman (98, 2, p. 512) has accepted this homology of the oviducts oi Cepha- 

 lodiscus. The consideration of the lateral mesenteries of Phoronis is, however, complicated by 

 the difference of opinion which exists as to the interpretation which should be given to the 

 mesentery which connects the oesophageal with the intestinal limb of the alimentary canal for 

 some distance below the region of the nephridia. It has been maintained that this is a remnant 

 of a dorsal mesentery, while it was suggested by Caldwell (82, p. 373) that it is due to the 

 secondar)' connexion of the intestine with the left lateral mesentery, which, nearer the lophophore, 

 resembles the corresponding right mesentery in passing from the oesophageal limb to the body- 

 wall (cf. Caldwell's diagram, p. 383). The question has recently been discussed by de Selys 

 LoNGCHAMPS (03, p. 18), who accepts Caldwell's view, pointing out that in Actinotrocha there 

 is no tracé of a dorsal mesentery, and (p. 22) that the lateral mesenteries are suspensory bands 

 of the nephridia. 



In Cephalodiscus the lateral mesenteries are suspensory mesenteries of the oviducts, but 

 one of their princii^al functions is to carry the ovarian vessels originating from the dorsal 

 vessel, which is contained in a dorsal mesentery. At their extreme dorsal end (figs. 149 — 152) 

 they are connected with the body-w^all of the metasome. They do not, however, originate from 

 the pharyngeal limb of the alimentary canal, but from the dorsal mesentery and dorsal vessel. 

 In view of the absence of the dorsal mesentery in Actinotrocha, this difference is perhaps not 

 fundamental. The question of their homology in Cephalodiscus and PJwronis appears to me to 

 turn on the propriety of regarding the oviducts of the fornier as comparable with the nephridia 

 of the latter. The resemblance in the position of the external apertures of the two sets of organs 

 is undoubtedly striking, and the organs are definitely compared with one another in two diagram- 

 matic figures given by Masterman (98, 2, figs. 16, 17). But the homology does not seem to 

 me to be definitely established, and on the contrary it must be remembered that the genital 

 ducts of Balanoglossus do not resemble nephridia. 



M'Intosh (88, p. 5) States that the "hyaline and often .semi-translucent tube" oï Phoronis 

 èïtskii "is finely and concentrically striated, — lajer upon layer of the hypodermic secretion 

 entering into its composition"; and further that it includes "many minute sponge-spicules, diatoms, 

 fragments of silex or accumulations of coarser sand-grains". It may be enquired whether there 

 is any relation between the constitution of the tubes of Phoronis and that of the coenoecium 



