MORPHOLOGY AND RELATIONS OF SIPHONOPHORA 19 



that part of Hadzi's (1944) Turbellarian theory of Cnidaria which derives Anthozoa from Turbellaria. 

 But then I have only a text-book knowledge of the Turbellaria, and if text-books on Cnidaria are any 

 criterion, they can be grossly misleading. 



SOMATOCYSTS AND BRACTS 

 Garstang (p. 143) boldly revived Schneider's contention that the Calycophoran somatocyst is homo- 

 logous with the cavity of the cap-shaped bract of a larval Physonect. But whereas, as Garstang says, 

 Schneider, Woltereck and Moser all realized this, they thought that the Calycophorae were primitive, 

 and that in the Physonect (1) the primitive Calycophoran bract had secondarily become separated from 

 the nectophore, (2) this separated nectophore had become the pneumatophore, and (3) the separated 

 somatocyst had become the Physonect bract. The true significance of the homology according to 

 Garstang is this : the Physonects are primitive and the independent primary bract of the Physonect 

 (e.g. the larval bract of Physophora) has become secondarily fused with the precocious nectophore 

 in the Calycophorae; and this shows that originally Calycophorae as well as Physonectae possessed 

 larval, locomotive bracts, reminiscent of the larval tentacles of hydroid actinulae. 



So Garstang put forward, and himself adopted what he called the illuminating view that the 

 original larva of the Siphonophore ancestors was not a planula but an actinula, in which, before the 

 typical actinuloid characters could be developed, precocious budding became established. 



It is quite impossible to be sure that the somatocyst of Calycophorae represents a larval actinuloid 

 tentacle at all and, in addition, that it represents a larval tentacle of the ancestor of a particular group 

 of gymnoblast hydroids within the Capitata. In ontogeny it always arises, in my view, as an integral 

 part of the precocious larval nectophore-bud, and may in reality have no more connexion with the 

 larva than that. It may, in fact, be an oil-storing organ developed late in phylogeny which now appears 

 early in ontogeny. I think that Garstang was probably right in believing that bracts were developed 

 from actinuloid tentacles, though another possible explanation is that larval bracts are precociously 

 developed adult bracts, developed late in phylogeny. 



If a close study is made of the bracts of Athorybia and Melophysa, and they are compared with the 

 larval bracts of Agalma elegans, and particularly of the larval and young stem bracts of A. okentt, 

 it will be seen that the stem bracts of Physonectae in general are probably all homologous with larval 

 bracts (see Text-figs. 8, 9 and 26). 



ORIGIN OF NECTOPHORES 



One of the most difficult questions to answer about siphonophore phylogeny has always been how 



nectophores were evolved. Judging by their simple nematocysts and tentacles, lack of nectophores, 



and structure of the pneumatophore, Cystonects are the most primitive group of Siphonophora. Long 



ago, Kawamura (1910) in an inaccessible * Japanese work, suggested a way in which the three-branched 



tentilla of Rhizophysa filiformis might have evolved from a simple unbranched tentacle. The process 



can be watched in the course of ontogeny. The great puzzle was how, if Cystonects are primitive, 



did the nectophores of Physonects originate ? Garstang linked his explanation that they represented 



pre-siphonophore (i.e. hydroid) gonophores with an argument that does not appear to be very sound, 



namely that hydroid gonophores were originally produced on the hydrorhiza of the ancestral hydroids, 



and that only later did they invade the circumoral region via the hydrocaulus with which he homo- 



logizes the siphonophore stem. 



Before proceeding with the discussion of Garstang's theory of the origin of nectophores it should 



be noted that Garstang made it somewhat difficult for a non-specialist to follow some of his arguments 



1 Through the good offices of Dr Mary Sears most of Kawamura's work on the Siphonophora has been translated from 

 Japanese into English by Rodney Notomi. 



3-2 



