MORPHOLOGY AND RELATIONS OF SIPHONOPHORA 25 



even be that calycophoran larvae have discarded, in favour of a precocious nectocalyx, a previously 

 possessed aboral float. He quite justifiably trounced the systematists for not having noticed the 

 significance of the atrophy of the anterior end of the larval planula in Calycophorae. Moreover, 

 although the early development of Hippopodiids is still not well known, 1 probably because it takes 

 place in deep water, Metchnikoff 's figures of early stages seem to indicate that the larval nectophore 

 of Hippopodius is in fact produced by the aboral (anterior) part of the planula. I reminded Garstang 

 of this some time after the publication of his paper, but by then he was approaching the end of his 

 life's work and felt unable to go into the matter again. 



The reduction in number and kind of appendages in the Calycophorae and their detachment in the 

 form of highly specialized, free-swimming eudoxid groups is almost certainly an evolutionary advance, 

 and their significance lies in the fact that this arrangement tends to increase the chances of cross- 

 fertilization with all its attendant advantages. Also, the fact that calycophoran species are so much 

 more numerous than physonect ones would suggest that the evolution of eudoxids had been followed 

 by deployment of the group. And judging by the long tubular nectosac of the posterior nectophore 

 of Ceratocymba dentata and the complex structure of the gonophores of its eudoxid, one would certainly 

 think that evolutionary progress had gone furthest in the Calycophorae. 



In considering which group is derivative, however, it has to be borne in mind that in Physonects 

 the polyps are of two kinds, as Vogt realized long ago, each with a specialized function. First, there 

 is the sterile gastrozooid with a large nematocyst factory (basigaster) for the highly evolved, branched 

 tentacle. Secondly, there are the sexual palpons (with minute basigaster and simple tentacle), some of 

 which bear at their base a prominence of varying length that carries the sexual medusae. This 

 prominence ('stalk of the gonophore ') is obviously not itself a polyp or gonopalpon because it has no 

 tentacle : it is part of the base of a palpon, and is quite short as a rule in the male gonophore. In the 

 Calycophorae, on the other hand, the gonophores generally arise from the base of polyps of the 

 highly evolved, gastrozooid type. Does the absence of the palpon-type of sexual polyp in the 

 Calycophorae mean that reduction has taken place, or does the presence of the gastrozooid type of 

 sexual polyp, with a highly evolved kind of tentacle mean that in this respect Calycophorae are an 

 advance on the Physonectae? 



LARVAE 



Since Garstang wrote his valuable thesis I have seen post-larvae of three forms related to Hippopodius 

 hippopus, namely Vogtia glabra, V. serrata— also figured by Moser (1925)— and Rosacea plicata (see 

 PP- 73-77)- A study of these has convinced me that the adult Nectopyramis diomedeae is a neotenous 

 form closely resembling in basic plan all these post-larvae. The Hippopodiids seem to have some 

 characters more primitive than those of Rosacea. 



CLASSIFICATION 



The classification given by Moser (19246) in Kukenthal's Handbuch almost completely obscures the 

 natural relationships of the Siphonophora and should not be followed. 



Libbie Hyman's text-book (1940) will be very widely used. Her classification of Siphonophora 

 (PP- 370, 475) is: Qrder SIPHONOPHORA 



Suborder CALYCOPHORAE 

 Suborder PHYSOPHORIDA 



Group PHYSONECTAE 

 Group RHIZOPHYSALIAE 

 Group CHONDROPHORAE 



1 I failed in my attempts at Villefranche to breed Hippopodiid larvae (see Systematic Notes, p. 77). 



