120 



COLLEGE ZOOLOGY 



Exumbrella 

 Mesoglea - 



Gastrovascular cavity 

 Subumbrella 



Ring canal 

 Nerve ring 

 Tentacle 



Velum 



Adhes 



Epidermis 



Gastrodermis 



-Manubrium 

 Radial canal 



— Gonad 



Mouth 



Oral lobe 



Statocyst 



Tentacular bulb 



OJI 



Figure 58. Diagram of a hydrozoan medusa with part cut away so as to show the internal 

 structure. Natural size, one inch in diameter. 



free-swimming, but soon becomes fixed to 

 some object and grows into a polyp. 



Physalia— a polymorphic 

 colonial hydrozoan 



Physalia is a colony of hydrozoan polyps. 

 A colony containing two kinds of individ- 

 uals is said to be dimorphic; one containing 

 more than two kinds, polymorphic. Some of 

 the most remarkable cases of polymorphism 

 occur among the Hydrozoa. Physalia, or the 

 Portuguese man-of-war (Fig. 59), for exam- 

 ple, consists of a gas-filled float (pneumato- 

 phore) with a sail-like crest, from which a 



number of polyps hang down into the water. 

 Some of these polyps are nutritive (gastro- 

 zooid), others are feelers ( dactylozooid ) , 

 and others are reproductive zooids (gono- 

 zooid ) . 



The surface of the float shimmers with 

 beautiful iridescent colors: blues, pinks, 

 violets, and purples, and the crest may glow 

 with vivid carmine. The different types of 

 zooids in Physalia arise from a single planula 

 larva and bud off from a section of the 

 coenosarc just beneath the float. This 

 strange animal occurs in the Gulf Stream 

 from Florida northward; specimens are often 

 cast up on the shore. It has no effective 



