26 THE VOYAGE OF H.M.S. CHALLENGER. 



with the modification of the nervous system. Moreover, the ganglion in the hood or 

 prseoral lobe of Actinotrocha has been lost in the metamorphosis. 



The peculiar organs situated above the mouth, and in connection with the second 

 branchial series, may possess a sensory function, though it is probable also that the 

 thickened hypoderm at the bases of the various branchial whorls may perform similar 

 service. The remarkable mesenteric (hypodermic-like) folds, and their extensions in the 

 spaces just behind the central nervous system, appear likewise to have allied functions. 

 It is probable that the functions of the former may be made more manifest by further 

 study. It seems to be much less developed in the young than in the adult. Should 

 internal apertures at any stage ever be found, their homologies with the collar-pores 

 of Cejyhalodisciis and Balanoglossus would be interesting. 



Body-Wall. — The structure of the body -wall externally approaches that of 

 Cej^halodisciis, both in regard to its thick glandular hypoderm, its basement-tissue, and 

 the abundance of its secretion for the formation of a tube. In some respects it also 

 resembles that of Balanoglossus. On the other hand, its muscular system shows a 

 great advance on that of Cephalodiscus, the stalk of the latter alone presenting similar 

 powerful bands. The somewhat pennate arrangement of the longitudinal muscular 

 bands in Phoronis quite diifers from that of its allies. 



The Body-Cavity appears to show only two divisions, an anterior and a posterior, 

 the latter having a corpusculated fluid in its interior. Phoronis would therefore seem 

 to have undergone considerable modifications. The posterior body-cavity is divided by 

 various mesenteries, and largely occupied by radiate muscular fibres and connective- 

 tissue. The connection of the vascular spaces in front with the tentacles may be held 

 as indicative of some affinity with the collar-spaces of Cephalodiscus, which (spaces) 

 can likewise be traced into the tentacles. There is room for further researches in this 

 region of the l:)ody at various stages of development, and especially the post-larval. 



In regard to the Reproductive Organs, Phoronis is better known than either 

 Rhcd)dopleura or Ce2)halodiscus, and the development and life-history have been more 

 or less completely outlined. Phoronis is conspicuously hermaphrodite. The interesting 

 resemblance of its larval form (Actinotrocha) to the larval form of Balanoglossus, 

 discovered by Mr. Weldon, have been alluded to by Mr. Sidney Harmer, and indicate 

 how complex the relationships of such forms are. The latter author thinks it probable 

 that since the oviducts of Cephalodiscus do not open into the body-cavity, the collar- 

 pores rather than these may be the homologies of the nephridia of Phoronis. If, 

 however, an aperture were found on each side into the body-cavity between the mouth 

 and the nephridia — even in the young animal — this view would require modification. 



On the whole, then, on comparing Phoronis with Cephalodiscus and its allies, we 

 miss the proboscis ; while the collar and trunk are more or less fused. There are no 

 gill-slits. The nervous system seems to be placed in a region probably homologous 



