230 BULLETIN OF THE 



whole head modified into the form of an elongated axis or stem. By these 

 changes the clusters of grape-like organs would appear as lateral branches of a 

 main stem ; and if we suppose the clusters of gonophores pushed out to their 

 tips, we should have an exact resemblance to the condition of the gonosomes * 

 of Hydrichthys, where they are simply botryoidal clusters of immature medusae 

 mounted on peduncles which arise from a common stalk. How is it with the 

 filiform bodies of Hydrichthys ? In reply, it may be said these do not occur 

 in Tubularia. Morphologically, they may be supposed to be the single simple 

 hydroid, stripped of tentacles, gonophores, and enveloping sheath, so that the 

 axis alone, with its terminal opening, is about all that remains. By this 

 reduction we have one of the simplest forms of hydroids. Such an individual 

 is certainly as low in organization as the Protohydra, Microhydra, and similar 

 low genera which are destitute of tentacles. 



This reduction in the form^of the hydranth by the disappearance of the ten- 

 tacles in Hydrichthys is believed to be a degeneration brought about by its 

 life, and not, as in Protohydra, due to the low zoological position of the hy- 

 droid. f The character of the medusa of Hydrichthys and its resemblance to 



* As this comparison is only in general external outlines, no account of the 

 fact that the gonophores of Tubularia take the form of actinulae, while those of 

 Hydrichthys appear as medusae, is considered. En passant, however, it might be 

 said that morphologically the actinula and the medusa are thought to be homolo- 

 gous, as several naturalists have already shown. I regard both medusa and hy- 

 droid as a modification in different directions of an ancestral form which is most 

 closely adhered to in a stage of the Siphonophores to which I have given the name 

 "primitive larva," or " primitive medusa." Morphologically considered, a medusa 

 and a simple hydroid are homologous, as shown by a study of Stephanoscyphus 

 ( Allman), Cunina, the young of Agalma as compared with the young Nanomia, and 

 other genera. This identity, in a morphological way, of medusa and hydroid has 

 long been recognized, and was pointed out many years ago by Claus and others. 



The egg in its development may pass into one or the other of these homologous 

 stages. It may become fixed to a submarine object, and become a fixed hydroid; 

 it may pass into a free medusa or medusiform condition homologous to a hydroid, 

 as in Glossocodon or in Agalma ; or it may be developed into a parasitic Hydrich- 

 thys. It seems probable that, as I have already elsewhere shown, the attached form 

 of the medusa or the hydroid is a secondary condition, and that the primary con- 

 dition is a direct development from the egg to the adult medusa. I would regard 

 the ancestral form of metagenesis to be the development of the " primitive me- 

 dusa," from an organism with both hydroid and medusan affinities, directly from the 

 egg without attachment. From that medusa, — which I would call and have else- 

 where named the primitive medusa, — in some instances, free medusiform gono- 

 phores bud, as in Agalma; in other cases, the primitive medusa becomes attached, 

 and is modified into a hydroid from which free gonophores separate ; while in still 

 other cases, Nanomia, the primitive medusa is neither medusiform nor attached 

 hydroid-like, but planula-like, with a float. The primitive medusa is homologous 

 in all these changed forms. 



t There is no reason to suppose that non-tentaculated genera allied to Hydra 



