POLYZOA. 



553 



of the ganglion, between the ganglion and the median dorsal part of the 

 ring-canal. In the Phylactolaemata this median dorsal part of the ring-canal 

 communicates with the exterior by a pore which together with some columnar 

 ciliated epithelium of the adjacent parts of the ring-canal constitutes the 

 so-called nephridium.* In some Gymnolaemata the intertentacular organ 

 opens externally in a corresponding position, and internally into the body-cavity. 

 Into what part of the body -cavity the intertentacular organ opens is not clear 

 from the accounts available, but presumably into the general body-cavity, for 

 it is said to form an escape-duct for the ova. 



The zooids are usually hermaphrodite. The testes are developed 

 either on the upper part of the funiculus, or near the point of 

 attachment of the latter to the body-wall. The ovaries are placed 

 on the body- wall near the anterior end of the zooecium. Both kinds 

 of generative cells are dehisced into the body-cavity. There do not 

 appear to be any special renal organs, 

 unless the intertentacular organ be such, 

 and the fine ciliated canal of the Entoproda. 



Many forms of Polyzoa present examples 

 of a well-marked polymorphism. In Zoobo- 

 tryon and its allies the joints of the stalk 

 represent a special form of individual ; they 

 have a considerable size and a simplified 

 organization, and serve as the ramified sub- 

 stratum on which the nutritive zooids are 

 placed. In addition there are here and 

 there joints of the roots, which, under the 

 form of tendril and stolon -like processes, 

 serve to attach the colony. The stalk of 

 KinetosMas is a modified stalk -zooid. In 

 addition to these stem- and root -forming 

 zooids, there are the peculiar avicularia 

 (Fig. 442) and vibracula (Fig. 443). These 

 are only found in the Cheilostomata. They 

 are both modifications of an ordinary zooe- 

 cium, with the operculum as the mandible 

 in the one case, and as the lashing filament 

 in the other. A fully-developed avicularium 

 resembles a bird's head ; the upper beak is hooked, and the lower 

 one, or mandible, is spiked. There is a strong adductor muscle 

 arising in the "head" of the avicularium, and inserted into the 



* Verworn, Z. f. w. Z., 46. Braem, Bibliotheca, Zoologica, Heft 6. Cori, 

 Z. f. w. Z., 55. Oka, Journal Coll. Sri. Japan, 8. 



Ovz 



Fio. 442. Biigula avicularia 

 (after v. Nordmann). Te 

 tentacular crown; retractor 

 muscle ; D alimentary canal; 

 F funiculus; Av avicularia; 

 Oes oesophagus ; Ovz ovicells. 



