HTDROZOA. 107 



the margin of the limb, to open on its dorsal sur- 

 face, where they secrete a clear, viscid, mucus. The 

 true nature of this mucus, whether excretory or 

 lubricative, is still very imperfectly known. 



Thus the most striking modifications of the 

 common plan of the Physo'phorldw depend on 

 differences in the relative size and shape of the 

 coenosarc and pneumatophore. Athorybia and 

 PhysaUa have, like Velella and Porpita, a dispro- 

 portionately large pneumatophore; but, in these ge- 

 nera, it is globular or pear-shaped, not, as in those, 

 discoidal. In PhysaUa, the true Portuguese Man- 

 of-war of sailors, often wrongly regarded as the 

 type of the present group, the float sometimes 

 attains a long: diameter of eio:ht or nine inches; 

 tentacles, several feet in length, being attached 

 directly to the coenosarc along its under surface 

 {jig. II, c). But, more frequently, the coenosarc 

 is filiform, with a small pneumatophore ; and, ex- 

 cept in the case of Rhizophysa, swimming-bells 

 are also present. Xectocalyces and hydrophyllia 

 are alike absent in Porpita, Velella^ Physalia, and 

 Rhizopjliysa. Athorybia has hydrophyllia, without 

 nectocalyces ; Physophora nectocalyces, but no 

 hydrophyllia. All other genera possess these two 

 kinds of appendages. 



The swimming-bells of the Physophoridce, when 

 present, are more numerous than in the Galycopho- 

 ridce, and, among the different genera, vary much 

 in size, shape, and mode of attachment, as also in 

 the relative proportions of the nectosac. Each fre- 

 quently has its surface marked with grooves and 

 ridges, and may send forth processes which serve 

 to embrace the coenosarc, and connect it with its 

 fellow of the opposite side. In some genera, more 



