514 



POLYZOA 



CHAP. XVIII 



body which is best fitted for ascertaining which is the propefl 

 substance on which to fix. 



Budding. The formation of a new individual may take 

 place by the outgrowth of part of the body-wall, as in Pedicellina 

 (Fig. 243, p. 487) and in Bowerhanhia (Fig. 238, p. 480). 

 In Pedicellina a young stalk is formed by an outgrowth near 

 one of the growing points, and the upper part of this outgrow tli 

 becomes constricted off to form the calyx. In other cases (cf. 

 the growing ends of the branches in Fig. 237) a partition 

 grows across the body-cavity at the growing edge of the colony, 

 and so cuts off a part destined to become a new zooecium. 



The zooecium formed in one of these ways acquires an 

 alimentary canal by the formation of a polypide-bud, some 

 stages in the growth of which are shown in Fig. 235 (p. 472). 

 Contrary to what happens in Coelenterates and . Tunica tes, in 

 which the endoderm takes part in the budding, there is good 

 reason for believing that in Polyzoa the polypide-bud is developed 

 entirely from ectoderm and mesoderm. 1 The bud is a two- 

 layered vesicle, attached to the inner side of the body-wall. Its 

 inner layer is derived from the ectoderm, which at first projects 

 into the body-cavity in the form of a solid knob surrounded by 

 mesoderm-cells. A cavity appears in the inner, ectodermic mass, 

 and the upper part of the vesicle so developed becomes exces- 

 sively thin, forming the tentacle-sheath, which is always de- 

 veloped in the condition of retraction. The lower part becomes 

 thicker ; its inner layer gives rise to the lining of the alimentary 

 canal, to the nervous system, and to the outer epithelium of the 

 tentacles, which grow out into the tentacle-sheath (cf. Fig. 235). 

 The outer layer gives rise to the mesodermic structures, such 

 as the muscles, connective tissue, and generative organs. 



These processes are fundamentally similar, whether in the 

 metamorphosed larva, in a young zooecium, in an old zooecium 

 after the formation of a " brown body," or in the germinating 

 statoblast of the Phylactolaemata. 



1 Cf. Seeliger, Zeitschr. iviss. Zool. xlix. 1890, p. 168 ; and 1. 1890, p. 560. 



