100 



PRINCIPLES OF ANIMAL BIOLOGY 



cither polyp or medusa. In certain species the bracts contain remains of 

 radial canals which are characteristic of medusae. The bracts, swimming 

 bells and gonophores are constructed on a medusoid plan while the feed- 

 ing polyps, sensory polyps and tentacles are constructed on the polyp 

 plan. In a few species the gonophores may separate from the colony, 

 as do the medusae in typical hydroids, but usually they remain attached. 



The "Portuguese man-of-war," Physalia (Fig. 65), differs from the 

 generalized form described above in possessing a float which sits high 

 above the water and serves as a sail. It has no swimming bells or bracts. 



Colonies in Other Metazoa.- — ^Colony formation occurs among a few 

 other groups of animals, namely, the Porifera, the Bryozoa, Cestoda, 



A B 



Fig. 66. — Structures of Bryozoa. A, Bugula aoicularia, portion of a colony; Av' 

 avicularia; D, alimentary c'anal; F, funiculus; Oes, esophagus; Ovz, ovicells; R, retractor 

 muscle; Te, tentacles. {From Sedgwick' a Textbook of Zoology, after von Nordmajin.) B, 

 Caberea ellisi, portion of a colony, fr. av, frontal avicularia; lat, av, lateral avicularia; 

 V, vibracularia; v. ch, vibracular chamber. (From Robertson in University of California 

 Publications.) 



Annelida (rarely), and Tunicata. Sponges (Porifera) are considered to 

 form colonies but the fusion of the bodies of the individuals is so complete 

 that it is difficult to distinguish their limits. The Bryozoa (moss animals) 

 form dendritic, spheroid, or irregular colonies. Some of them, for ex- 

 ample Bugula, also exhibit polymorphism. In a colony of Bugula the 

 most common individual is the nutritive zooid provided with tentacles. 

 Of the other members of the colony the avicularia (Fig. 6G, A), a type of 

 individual shaped like a bird's head and having a movable jaw-like struc- 

 ture, may serve as grasping devices, perhaps for defense; and ovicells are 



