72 EMBRYOLOGY 



stant replacement by reserve nectocalyces of the same form. This 

 replacement also plays a considerable role, as we shall see directly, 

 in the metamorphosis of the Calycophoridse. 



The metamorphosis of the Caljcophoridse has been made 

 known chiefly through the investigations of Chun (No. 54). 

 These refer prinoipallj to the development of the Mono- 

 phjida?, i.e. those forms which are characterized by the 

 possession of a single nectocalyx on the nectosome. In a 

 small Monophyid called by Chun Muggigea Kochii, and 

 characterized by its tall pentagonal nectocalyx, Chun was 

 able to prove that the larvae arising from the eggs at first 

 possess a quite differently shaped, cap-like nectocalyx. By 

 the casting off of this primary provisional nectocalyx and 

 its replacement by the permanent heteromorphous one, 

 these larva3, designated as Monophyes primordialis, pass into 

 the Muggiaea form, from the stem of which the groups of 

 individuals at sexual maturity detach themselves as Eudoxia 

 Eschscholtzii. 



Since Chun has recently been able to prove the presence 

 of these primary, heteromorphous, deciduous nectocalyces 

 in the case of the Polyphyidse, it may be considered probable 

 that such nectocalyces also belong to the larval stages of all 

 Calycophoridae. According to Chun's theory, which Haeckel 

 has adopted, the fundament of the pneumatophore in the 

 Physophoridee would be homologous to the deciduous, pri- 

 mary nectocalyx of the Calycophoridae. 



General Considerations.— As regards the derivation of 

 the Siphonophora, there are at present two views, as yet 

 directly opposed to each other, underlying both of which is 

 the conception that the Siphonophore is a polymorphous 

 animal stock that has arisen by budding. But while some 

 authors (Leuckart, Claus, Chun) assume the starting-point 

 of this stock to be a floating hydroid-polyp stocklet, which 

 already had the power of producing medusae {hydroid theory), 

 others (Balfour, Haeckel) derive the Siphonophore from a 

 medusa, which, by the budding of its manubrium (like Sarsia 

 or Hybocodon), was able to produce new medusse {medusa 

 theory). The former authors accordingly have two funda- 

 mental forms from which they are able to derive the mani- 



