SYSTEMATIC AND BIOLOGICAL ACCOUNT 75 



We are now in a position to see how closely alike are the larvae of two species of Prayidae and of 

 three species of Hippopodiidae, for Chun (18880, b, 1913) had already figured three larvae of Hippo- 

 podius hippopus. In all except Rosacea cymbiformis there is a deep, pocket-shaped hydroecium, and in 

 none is there an extension of the somatocyst distad to the pedicular canal of the nectosac. All, except 

 Vogtia glabra, where they are missing, have simple straight, lateral subumbral canals such as are 

 found in Desmophyes. Reference to Bigelow's (191 \b) figure of Nectopyramis diomedeae will show 

 that these same features appear in the definitive nectophore of the species, which I consider to be 

 a good instance of a neotenous form which has subsequently evolved on new lines. We do not, of 

 course, know whether the common ancestor had a very different adult stage. Text-fig. 34 will make 

 this discussion plain. 



CpeoJ 



N.detl 



Nsdefi- ■ 



Nlorv- - 



Text-fig. 33. Vogtia glabra. Prof. Tregouboff's larva from Villefranche, x 19-5. For photograph, see PI. IV, fig. 2. 



It may be mentioned at this point that the definitive nectophores of Hippopodiid species resemble 

 the larval type common to Hippopodiidae and Prayidae in that there is no distal prolongation of the 

 somatocyst beyond the pedicular canal of the nectosac. 



There is present in Prayid and Hippopodiid larval nectophores an ovate or globular structure that 

 lies between the pallial canal (somatocyst) and the gastrozooid. In older stages it is transformed or 

 absorbed into the stalk canal that traverses the muscular lamella. It has been considered that the 

 free end of the pallial canal in Prayids is homologous with the somatocyst in other Calycophorae, but 

 the origin of the rest of the pallial canal is in doubt. Metchnikoff 's figures of Hippopodius hippopus 

 larvae show no sign of a homologue of the somatocyst, which appears so clearly in all other 

 Calycophorae. The stalk canal of the larval nectophore (not the pedicular canal of its nectosac) must 

 lie somewhere near the morphologically aboral end of the post-larva, and the curved pallial canal, so 

 characteristic of Prayids, has probably arisen as an endodermal proliferation at the junction of the 

 yolky larva proper and its enormous mesogloeal nectophore. This pallial canal terminates at the inner 



