iv PHYLUM CCELENTERATA 163 



become further and further apart, being always situated at the 

 distal and proximal ends of the colony respectively. 



In an allied form (Acjalma) the first structure to appear in the embryo is not 

 the float, but the first bract, which grows considerably and envelops the growing 

 embryo in much the same way as the umbrella of a medusa envelops the manu- 

 brium. On this and other grounds some zoologists look upon the Siphonophore- 

 colony as a medusa the manubrium of which has extended immensely and 

 produced lateral buds after the manner of some Anthomedus* (Fig. 105, 7 a). 



FIG. 121. Two stages in the development of Halistemma : the endoderm is shaded, the 

 ectoderm left white, ep. pneumatocyst or air-chamber of pneumatophore ; /*?/. endoderm 

 surrounding pneumatophore ; po. polype ; pp. pneumatophore ; t. tentacle. (From Balfour, 

 after Metschnikoff.) 



On this theory the entire comosarc is an extended manubrium, and the first or 

 primary bract is the umbrella. But frequently as in Halistemma a primary 

 bract is not formed, and when present there appears to be no reason against 

 regarding it as a lateral bud of the axis, of quite the same nature as the remaining 

 zooids. 



In the well-known " Portuguese man-of-war " (Physalia) there 

 is a great increase in proportional size of the float and a corre- 

 sponding reduction of the rest of the ccenosarc. The float (Fig. 

 122, pn) has the form of an elongated bladder, from 3 to 12 cm. 

 long, pointed at both ends, and produced along its upper edge 

 into a crest or sail (cr) : as a rule it is of a brilliant peacock-blue 

 colour, but orange-coloured specimens are sometimes met with. 

 At one end is a minute aperture communicating with the exterior. 

 There are no swimming-bells, but from the underside of the float 

 hang gastrozooids (p), dactylozooids, branching blastostyles 

 (gonodendra) with groups of medusoids looking like bunches 

 of grapes of a deep blue colour, and long retractile tentacles (t), 



M 2 



