XI SIPHONOPHORA PHYSOPHORAE 309 



the large poly thalamic pneumatophore and the single large central 

 gastrozooid. 



The colony is disc-shaped, and has a superficial resemblance 

 to a Medusa. On the upper side is the flattened pneumatophore, 

 covered by a fold of tissue continuous with that at the edge of 

 the disc. In Velella a vertical triangular sail or crest rises from 

 the upper side, but this is absent in Porpita. 



The mouth of the gastrozooid opens into a large digestive 

 cavity, and between this and the under surface of the pneumato- 

 phore there is a glandular spongy tissue called the liver. The 

 liver extends over the whole of the under side of the pneumato- 

 phore, and sends processes round the edge of the disc into the 

 tissues of its upper surface. Intimately associated with the 

 liver, and penetrating its interstices, is an organ which appears 

 to be entirely composed of nematocysts, derived from the ectoderm, 

 and called the central organ. At the margin of the disc there 

 is a fringe of simple digitiform dactylozooids, and between the 

 dactylozooids and the centrally placed gastrozooid are numerous 

 gonozooids. Each of the gonozooids is provided with a distinct 

 mouth, and bears the gonophores, which escape before the ripen- 

 ing of the gonads as the free-swimming Medusae called Ghryso- 

 mitra. The pneumatophore consists of a number of annular 

 chambers arranged in a concentric manner round the central 

 original chamber formed from a modified zooid. These annular 

 chambers are in communication with one another, and have each 

 two pores (pneumatopyles) opening above to the exterior. The 

 most remarkable feature, however, of the system is a series of 

 fine branching tubes (" tracheae "), which pass from the annular 

 chambers of the pneumatophore downwards into the hepatic 

 mass and ramify there. 



There are two well-known genera: Velella with a sail, and 

 Forpita without a sail. They are both found at the surface of 

 the warmer regions of the great oceans and in the Mediterranean. 

 Velella sometimes drifts on to British coasts from the Atlantic. 



The genus Discalia has a much more simple octoradial 

 structure. It was found at depths of 2600 and 2750 fathoms 

 in the Pacific Ocean. 



