54 



MORPHOLOGY. 



between the marsupial receptacles of Sertularia rosacea and Sertularia fallaj; on the one hanrl, 

 and Sertularia tamarisca on the other, are at once apparent, and are very interesting. The 

 ramified tubes of S. tamarisca are manifestly the representatives of the simple tubes in S. rosacea 

 and S.fallax; while the three broad chitinous leaflets within which the ramified tubes are con- 

 tained are homologous with the hollow chitinous processes of the gonangium, which in S. rosacea 

 and S.fallax enclose the simple tuljes, and subsequently coalesce to form with their prominent 

 ridges and spines an external capsule-like covering for the sac, into which, as in S. tamarisca, the 

 ova are expelled from the gonangium proper. 



The structures just described in Sertularia rosacea, S.fallax, and S. tamarisca will, I think, 

 enable us to explain a peculiar feature observed in aS*. pumila, and probably some other species. In 

 S. pumila the blastostyle of both male and female gonangia gives off from its enlarged opercular 

 summit several more or less ramified esecal tubular processes (woodcut, fig. 21, d, d), which, 

 instead of developing themselves externally, are found entirely within the gonangium, where they 

 hang freely from the summit of the blastostyle. Their walls are composed both of endoderm and 

 ectoderm, and their cavity communicates with that of the blastostyle, so that the peculiar coloured 

 corpuscles which circulate within the cavity of the blastostyle are freely admitted into the esecal 

 tubes, where they may occasionally be seen in active motion. The tubes can be most satisfactorily 

 examined in the younger gonangia. In the older ones they will frequently be found to have 

 contracted adhesions to the gonangium, to have become atrophied, and, finally, even to have 



disappeared. 



I believe that these tubes are the exact equivalents 



of those which in Sertularia tamarisca and S. rosacea are 

 given off from the same part of the blastostyle, but where, 

 instead of growing into the cavity of the gonangium, they 

 are developed in an outward direction, and assist in the 

 formation of the peculiar receptacle which surrounds the 

 acrocyst in those species.' 



Among the most remarkable modifications of the tro- 

 phosome is that of Cojjpinia arcta (woodcut, fig. 27). In this 

 singular hydroid the hydrothec;e and gonangia springdirectly 

 from a creeping retiform hydrorhiza, while the gonangia, 

 which are very numerous, become closely adherent to one 

 another by their sides, so as to form with the jjroximal por- 

 tion of the hydrothecac and with the hydrorhiza a continuous 

 encrusting basis spreading over the surface to which the 



Fig. 27. 



A portion of the hydrosoma of Cophinia arcla. 



a, a, a, HydrothecEE ; b, a hydranth retracted 

 and destitute of tentacles; c, a hydranth re- 

 tracted and with its tentacles present; d, d, 

 basal eucrnsting portion formed by the juxta- 

 position and adhesion of tubular gonangia ; 

 e, sporosac, enclosing a solitary ovum ; f,f, 

 acrocysts, enclosing the ovum in a more ad- 

 vanced stage of development; g, retiform 

 stolon, forming the hydrorhiza. 



respects from my original description of the same part {' Brit. 

 Assoc. Report on Hydroid.\'), subsequent more favorable ex- 

 aminations havinar caused certain modifications of mv former 



' It is evidently the tubes here described to which Agassiz 

 (' Nat. Hist. U. S.,' vol. iv, p. 329, pi. xxxii, figs. 10, 10") refers 

 as occurring in a North American hydroid which he regards as 

 identical with the Sertularia pumila of the European coasts. 

 He views them, however, as simply representing the fleshy 

 bauds which may frequently be seen iu the trophosome of the Hydroida, exteudiug from the outer 



