72 INVERTEBRATE ANIMALS. 



PhysophoridcB (Gr. physa, a bladder; and phero, I carry), of 

 which the most familiar, though not the most typical, example 

 is the Portuguese man-of-war, Physalia utriculus (Fig. 20, a). 

 The Physophoridce are distinguished from the organisms 

 which we have been just considering by the fact that one 

 extremity of the ccenosarc is developed into a structure which 

 is known as the " float " or " pneumatophore " (Gr. pneuma, 

 air; and phero, I carry). The float contains a larger or 

 smaller sac, composed of some elastic, horny substance, proba- 

 bly chitine, often communicating with the exterior by one or 

 more apertures, and always more or less completely filled with 

 air. This sac is enclosed in a reflection of the ectoderm and 

 endoderm, so that it is really outside the cavity of the cceno- 

 sarc. The function of the float is no doubt that of enabling 

 the organism to maintain its position at the surface of the 

 sea. As in the Calycophoridce, the ccenosarc is always per- 

 fectly flexible, contractile, and soft, and is never furnished 

 with any chitinous covering or poly pa ry. There may or may 

 not be swimming-bells, and the tentacles are very complicated 

 in structure, and often attain a length of many inches. The 

 polypites present no special points of interest, but are often 

 furnished with the protective plates, which have been already 

 spoken of as " bracts." 



As a good example of the Physophoridce, the Portuguese 

 man-of-war may be taken (Fig. 20, 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 floating at the 

 surface of tropical and sub-tropical seas, and fleets of it are 

 occasionally driven upon temperate shores. 



Another very beautiful member of the Physophoridce is 

 the Velella vulgaris, which occurs abundantly in many seas. 

 It is about two inches in length by one and a half in height. 

 One end of the ccenosarc is greatly expanded and flattened out 

 into an oval disk, which carries a vertical triangular crest, 

 running obliquely across its upper surface (Fig. 20, b). The 

 whole organism is semi-transparent and of a beautiful bluish 

 color, and it floats at the surface of the sea with the vertical 

 crest exposed to the influence of the wind, and thus officiating 

 as a sail. From the under surface of the disk are suspended 

 the various appendages of the organism, consisting of a single 



