SIPHONOPHORA 30I 



This Ijladder or pneumatopliore is probably in all cases a much 

 luodified nectocalyx. It shows great variations in size and 

 structure in tlie group. It is sometimes relatively very large, as 

 in Physalia and Vddla, sometimes very small, as in Fhysophora. 

 It is provided with an apical pore in some genera {Rhizophysa), 

 or a basal pore in others (Auronectidae), but it is generally closed. 

 In the many cliambered pneumatophore of the Chondrophoridae 

 there are several pores. 



In many forms two distinct parts of the pneumatophore can 

 be recognised — a distal region lined by chitin,^ probably repre- 

 senting the sub-umbrellar cavity of the nectocalyx, and a small 

 funnel-shaped region lined by an epithelium, the homology of 

 which is a matter of dispute. It is believed that the gas 

 is secreted by this epithelium. In the Auronectidae the region 

 with secretory epithelium is relatively large and of a more 

 complicated histological character. It is remarkable also that in 

 this family the pore communicates, not with the chitin-lined 

 region, but directly with the epithelium-lined region. 



There is no pneumatophore in the Calycophorae, but in 

 this sub-order a diverticulum of an endoderm canal secretes a 

 globule of oil which may serve the same hydrostatic function. 



The stolon is the common stem which supports the 

 different zooids of the colony. In the Calycophorae the stolon 

 is a long, delicate, and extremely contractile thread attached at 

 one end to a nectocalyx, and bearing the zooids in discontinuous 

 groups. These groups of zooids arranged at intervals on the 

 stolon are called the "cormidia." The stolon is a tube with 

 very thick walls. Its lumen is lined by a ciliated endoderm 

 with circular muscular processes, and the surface is covered with 

 an ectoderm, also provided with circular muscular processes. 

 Between these two layers there is a relatively thick mesogloea 

 showing on the outer side deep and compound folds and grooves 

 supporting an elaborate system of longitudinal nmscular fibres. 

 In many Physonectidae the stolon is long and filamentous, but 

 not so contractile as it is in Calycophorae, but in others it is 

 much reduced in length and relatively stouter. Tlie nnluction 



1 The chemical composition of the substance liere called "chitin" lias not been 

 accurateh^ determined. An analysis of two specimens of Vclella liladders gave 9 "71 

 and 10 '35 per cent of nitrogen, which is higher than that of chitin and nearer to 

 that of mucin. 



