126 



COLLEGE ZOOLOGY 



e. Polymorphism 



The division of labor among the cells of a METAZOON has al- 

 ready been noted (p. 74). When division of labor occurs among 

 the members of a colony, the form of the individual is suited to 

 the function it performs. A colony containing two kinds of 

 members is said to be dimorphic; one containing more than two 

 kinds, polymorphic. Some of the most remarkable cases of 

 polymorphism occur among the HYDROZOA. The " Portuguese 

 man-of-war " (Fig. 77), for example, consists of a float with a 

 sail-like crest from which a number of polyps hang down into the 

 water. Some of these polyps are nutritive, others are tactile; 

 some contain batteries of nematocysts, others are male repro- 

 ductive zooids, and still others give rise to egg-producing me- 

 dusae. 



Tables V and VI present briefly the various modifications 

 that may occur among the members of colonial HYDROZOA. 



TABLE V 



POLYMORPHIC MODIFICATIONS OF THE MEDUSOIDS OF THE HYDROZOA 



NAME 



STRUCTURE 



FUNCTION 



Sexual medusoid 



Nectophore 



Hydrophyllium 

 Pneumatophore 

 Aurophore 



Like typical medusa of An- 

 thomedusse (p. 128) or 

 modified because of ar- 

 rested development (Fig. 



78, 



Without tentacles, manu- 

 brium, and mouth (Fig. 

 78, A-) 



Shield shaped (Fig. 78, /) 



Air sac (Fig. 78, n) 



Ovoid 



Production of ova or 

 spermatozoa 



Locomotion 



Protective 



Hydrostatic 



Unknown 



