SUB-CLASS SIPHONOPIIOEA. 75 



chitinous covering or polypary. There may or may not be swim- 

 ming-bells or nectocalyces, and the tentacles are very complicated 

 in structure, and often attain a great length. The polypites present 

 no special points of interest, but are often furnished with the pro- 

 tective plates which have been already spoken of as "bracts.'' 



As a good example of the Physophorida;, the Portuguese Man-of- 

 war may be taken (fig. 40, a). It is composed of a large spindle- 

 shaped float, often of several inches in length, upon the under 

 surface of which are arranged a number of polypites, together with 

 highly contractile tentacles of great length, and reproductive organs. 

 The tentacles are richly furnished with thread-cells ; and it has the 

 power of stinging very severely. Physalia is commonly found float- 

 ing at the surface of tropical and sub-tropical seas. 



Another veiy beautiful member of the Physophorida is the Yelella 

 vulgaris, which occurs abundantly in many seas. It is about two 

 inches in length by one and a half in height. The proximal end of 

 the coenosarc is greatly expanded and flattened out into an oval disc, 

 which carries a vertical triangular crest, running obliquely across 

 its upper surface (fig. 40, h). The whole organism is semi-transpa- 

 rent and of a beautiful bluish colour, and it floats at the surface of 

 the sea with the vertical crest exposed to the influence of the wind, 

 and thvis officiating as a sail. From the under surface of the disc 

 are suspended the various appendages of the organism, consisting of 

 a single large central polypite ; a number of processes, like polypites 

 in shape, and carrying medusiform gonophores ; and lastly, a single 

 series of tentacles which arise from the coenosarc quite independently 

 of the polypites. 



