Miscellaneous, 38& 



tinned by a narrow neck towards the summit of the proper capsule, 

 with whose cavity it would seem to communicate ; but the author 

 did not succeed in tracing its connexions beyond this point. 



These ova, with their investing sacs, and the surrounding csecal 

 tubes, would tlius lie entirely exposed, were it not that they are sur- 

 rounded by the three leaflets already mentioned as constituting the 

 trihedral portion of the gonophore. These leaflets are given ofi'from 

 the oval portion or proper capsule near its summit, and, being in con- 

 tact by their edges, completely enclose a space which is occupied by 

 the structures just described. 



These structures are thus truly extra-capsular, and correspond 

 with the extra-capsular ovigerous sacs which occur in Sertidaria pu- 

 mila, S. cujrressina, and other species, and into which the ova are 

 conveyed from the interior to undergo, as in a sort of marsupium, a 

 further development previously to their final liberation as embryos. 



With regard to the true import of the sporosacs and their relation 

 to the Medusoid buds produced by other Hydroids, the author in- 

 sisted on the necessity of bearing in mind that the spadix has no 

 ectodermal covering, and consists of endoderm alone. He considered 

 it to be homologous with the manubrium ("peduncle") of a Medusa 

 separated from its ectoderm by the intervention of the generative 

 elements, which in the sporosac are always found between the endo- 

 derm and ectoderm of an organ strictly homologous with the so-called 

 " peduncle " of a Medusa. By the continued growth of the generative 

 elements, the ectoderm is separated more and more from the endo- 

 derm, which now constitutes a diverticulum from the cavity of the 

 blastostyle, enveloped by the ova or spermatozoa, while the ectoderm 

 forms the walls of a sac which immediately confines these elements. 

 The whole is enclosed in an external sac, which seems to be an ex- 

 tension of the ectoderm of the blastostyle. 



"We have thus, in the sporosac of SertnJaria tamarisca, an organ 

 which easily admits of comparison with the Medusoid buds of other 

 Zoophytes ; it consists, in fact, of a manubrium peculiarly modified, 

 so as to constitute a sac for the retention of the generative elements, 

 and chiefly differs from the proper Medusoid buds in the non-deve- 

 lopment of a swimming-organ or umbrella. In other instances 

 {^CordylopJiorci, &c.), as the author has elsewhere shown*, peculiar 

 caecal tubes, generally more or less blanched, are developed from the 

 base of the s])adix, and thence extend, along with the ova or sperma- 

 tozoa, between the ectoderm and endoderm towards the summit of 

 the sporosac. The author had already compared these tubes to the 

 radiating gastro-vascular canals of a INIedusa ; and if this comparison 

 be just, they remain in the sporosac as the sole representatives of the 

 parts found in the umbrella of a Medusa f. A change of position, 



♦ Phil. Trans. 1853. 



t In a paper by the author, on Cordylophora /acM5fm(Phil.Trans.]853), 

 he expressed his belief that the umbrella of a Mednsa had its representative 

 in the walls of the sporosac ; subsequent examination, however, of the 

 sporosacs in a great number of species had caused him to modify this view, 

 and adopt that contained in the present communication. 



