iiS 



04, p. qy) of a general nerve-plexus in ihe epidermis of the entire lophophoral region. Sent ltz 

 03, 1, p. 406) finds that in regenerating specimens of Phoronis the central nervous system 

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

 of \'ertebrates 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 

 invag-inated cranofHon as an adult ortran 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 Ccplialodisais 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 Phoronis. Masterman (98, 2, p. 512) has accepted this homology of the oviducts of Ccpha- 

 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 clue to the 

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

 resembles the corresponding right mesentery in ])assing from the oesophageal limh to the body- 

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

 LoNGCHAMPS (03, ]). 1 8), who accepts Caldwell's view, pointing out that in Actinotrocha there 

 is no trace 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 principal 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-wall of the metasome. They do not, however, originate from 

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

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

 fundamental. The question of their homology in Cephalodiscus and J'horonis appears to me to 

 turn on the propriety of regarding the oviducts of the former as comparable with the ne}ihridia 

 of the latter. The resemblance in the position of the external a|)enures of the two sets of organs 

 is undoubtedly striking, and the organs are definitely compared witii one another in two tliagram- 

 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. 



INMntosh (88, p. 5) states that the "hyaline and often .semi-translucent tube" oi Phoronis 

 buskii "is finely and concentrically striated, — la\er upon la\er of the hypodermic secretion 

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

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

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



