18 DISCOVERY REPORTS 



Kramp does not appear to have produced arguments of similar worth for concluding that evolution 



has proceeded in the reverse direction, nor did he anywhere refer to Garstang's work, as I have already 



mentioned. 



ACTINULOID ANCESTORS 



The question whether it is theoretically impossible for the ancestral Metazoa to have been pelagic 

 is of critical importance. 



A morphological character possessed by so many of the Corymorphidae is the presence on various 

 parts of the stem of papillae and root-processes, which bear suggestive signs of homology with the 

 larval tentacles of actinuloid animals. What are we to conclude from the fact that so many groups 

 of Hydrozoa have actinuloid larvae — Trachymedusae, Narcomedusae, Chondrophora, (?) Siphono- 

 phora, Margelopsidae (Margelopsis haeckeli, Pelagohydra, Climacocodon), Corymorphidae, Myriothe- 

 lidae, Tubulariidae, and Corynidae and allies? It must mean that the common ancestor of all these 

 animals had an actinuloid larva. Garstang appears to have believed that, as in the Tunicata, this 

 larva was a purely locomotive one, that it was followed by fixation, and then by an adult stage of feeding 

 and growth, with budding last of all, but that in some forms fixation was postponed and finally 

 abandoned altogether. I should like to think that the adult ancestor had always been planktonic, 

 like the ancestral larva, and that fixation did not take place until much later in phylogeny. According 

 to Carter (1949) who briefly reviewed Hadzi's (1944) Turbellarian theory of the Cnidaria, Hadzi 

 believes that ' only the smaller Protista can float without either an elaborate structure adapted to enable 

 them to do so or considerable expenditure of energy. We must think, then ', he says, ' of the ancestral 

 metazoan as a bottom-living form, and not as a floating spherical colony. ... It would develop an 

 intermediate cell-layer, a parenchyma, between the ectoderm and the endoderm, to give solidity and 

 support to the body.' But if animals like the trachymedusan Liriope, whose youngest stages show 

 neither elaborate structure, except a mesogloea, nor appear to be obliged or able to expend con- 

 siderable energy, can live at the surface, it would seem that this argument is not valid. 



The Trachylina are thought to be the most primitive Cnidaria. It is true that most Trachymedusae 

 are well-marked bathypelagic forms, as are the Narcomedusae from northern latitudes, but in the 

 warm seas the latter suborder are found in the surface layers. 



POLYP AND MEDUSA 

 The argument for thinking of the medusa as the original adult form of Cnidaria rather than as 

 a dispersal device was stated by Brooks (1886) following Bohm (1878) and Claus (1878). He advanced 

 a theory that the polypoid generation in Hydromedusae is a persistent larval form, and that the adult 

 ancestral hydrozoan was some sort of medusoid, perhaps (see Brooks, 1886, pi. 41, figs. 6-7) rather 

 like the earliest medusa-stage of development of the trachymedusan Liriope, the adult stage of which 

 has made only a slight advance beyond its larval polypoid stage (Brooks, fig. 8). The ancestral poly- 

 poids, as suggested by Brooks and supported by Libbie Hyman, developed the habit of budding 

 before some of them took up a prolonged attached existence as polypoid oozooids or colonies. After 

 this, instead of becoming metamorphosed directly into a medusoid, the actinuloid produced the 

 medusoid by budding. Libbie Hyman (1940) pointed out that this view harmonizes with the develop- 

 ment of Siphonophores, where the planula develops into the polypoid and buds off the medusoid. 



LIBBIE HYMAN ON PHYLOGENY 

 Libbie Hyman, though repeating some outmoded theories about the interpretation of the morphology 

 of Siphonophora, has given a more satisfactory account (pp. 636-40) of the probable phylogeny of the 

 various groups of Cnidaria and Ctenophora than is found elsewhere. I find it hard to believe in 



