iv PHYLUM CLELENTERATA 105 



colony drifts about, completely at the mercy of wind and 

 tide, buoyed up by a bladder-like float or pneumatophore 

 containing air. Such a passively floating form is the 

 Portuguese Man-of-war {Physalia) (Figs. 48, 49) which has 

 an elongated float, pointed at the ends, and produced above, 

 along its upper edge, into a crest or sail {cr.). At one end 

 is a minute aperture communicating with the exterior. 

 From the under side of the float hang polypes (/), feelers, 

 groups of medusa-buds looking like bunches of grapes of 

 a deep blue colour, and long retractile tentacles, sometimes 

 several feet in length, and containing batteries of stinging- 

 capsules powerful enough to sting the hand as severely as a 

 nettle. The male reproductive buds remain attached and 

 take the form of sporosacs, while the female buds apparently 

 become detached as free medusae. Physalia arethusa is 

 common in the West Indies, and, borne northward by the 

 Gulf Stream, is occasionally met with on the coast of 

 southern New England, and off Nova Scotia. 



In such a Siphonophoran as Halistemma (Fig. 50), on 

 the other hand, there is a long, slender, flexible stem or 

 ccenosarc, at the upper end of which is a comparatively 

 small float. Next to this come a number of closely set, 

 transparent structures {net), having the general characters 

 of unsymmetrical medusae without manubria, each being a 

 deep, bell-like body, with a velum and radiating canals. 

 During life these swimming-bells or nectocalyces contract 

 rhythmically, — i.e., at regular intervals, — thus serving to 

 propel the entire organism through the water. Below the 

 last nectocalyx the character of the structures borne by the 

 stem changes completely : they are of several kinds, and 

 are arranged in groups which follow one another at regular 

 intervals. 



Some of these are unmistakable polypes (/) differing, 



