CGELENTERATA : HYDROZOA. 93 



embryos are ciliated and free-swimming, but ultimately fix 

 themselves, and develop into the plant-like colony^ from which 

 fresh medusoids may be budded off: The ova in the medusi- 

 form gonophores are usually developed in the course of the 

 gonocalycine canals, and not between the ectoderm and 

 endoderm of the manubrium, as is the case in the Corynida. 

 Examples of the order are Campdnularia Laomedea, &c The 

 distinctions between the Sertularida and Campanidarida are 

 certainly insufficient to justify their being placed in separate 

 orders. If united together, it would probably be best to adopt 

 the name Thecaphora (Hincks) for the order, and to employ 

 the names Sertularida and Campanularida for the sub-orders. 



CHAPTER IX. 

 SIPHONOPIIORA. 



Sub-class II. Siphonophora. — The members of this sub- 

 class constitute the so-called " Oceanic Hydrozoa ; " and are 

 characterised by the possession of a "free and oceanic hydro- 

 soma, consisting of several polypites united by a flexible, contrac- 

 tile, unbranched or slightly-branched ccenosarc, the proximal end 

 of which is usually furnisJud with ' nectocalyces,' and is dilated 

 into a 'somatocyst' or into a ' pneumatophore.' " — (Greene.) 



All the Siphonophora are unattached, and permanently free, 

 and all are composite. They are singularly delicate organisms, 

 mostly found at the surface of tropical seas, the Portuguese 

 man-of-war (Physalia) being the most familiar member of the 

 group. The sub-class is divided into two orders — viz., the 

 Calycophoridce and the Physophoridce. 



Order I. Calycophorid^. — This order includes those 

 Siphonophora whose hydrosoma is free and oceanic, and is pro- 

 pelled by '■'■ nectocalyces" attached to its proximal end. The hydro- 

 soma consists of several polypites, united by an unbranched cceno- 

 sarc, which is highly flexible and coiitractile, and never develops a 

 hard cuticular layer. The proximal end of the hydrosoma is 

 modified into a peculiar cavity called the " somatocyst." The re- 

 productive organs are in the form of medusiform gonophores pro- 

 duced by budding from the peduncles of the polypites. 



In all the Calycophoridce the ccenosarc is filiform, cylindrical, 

 unbranched, and highly contractile, this last property being due 

 to the presence of abundant muscular fibres. " The proximal 



