IV PHYLUM CCELElSfTERATA 163 



become further and further apart, being always situated at the 

 distal and proximal ends of the colony respectively. 



In an allied form {ArjaJma) the first structure to appear in the embryo is not 

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

 embryo m 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 tlie manubrium of which has extended immensely and 

 produced lateral buds after the manner of some AnthomedusEe (Fig. 105 7 a). 



biG. 1-1. — Two stages in the development of Halistemma : the endoderm is shaded, tho 

 ectodemi left whitL-. up. pneumatocyst or air -chamber of pneumatophore ; hy. endoderm 

 sun-ounding pneumatophore ; po. polype ; p'p. pneumatophore ; t. tentacle. (From Balfour, 

 after Metschnikoff.) 



On this theory the entire canosarc 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 coenosarc. 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 



