DEVELOPMENT OF THE POLYPIDE. 



37 



modified glandular ectodermal cells at the posterior pole (Fig. 



18 ^, x) — sucker-rudiment of Ostroumoff. The projection -which 

 carries the polypides is evaginated from the mantle -cavity, the 

 mantle-fold (/") at the same time bending back (Figs. 18 i? and 



19 B). The posterior pole of the body now becomes detached from 

 the substratum, the edges of the mantle-fold (/) approach each 

 other till they fuse (Fig. 18 C), and in this way a closed sac forms 

 in which is included the greater part of the external ciliated surface 

 of the larva, which then undergoes degeneration. 



The essential distinction between the metamorphosis here described 

 and that of the marine forms consists in the absence of the sucker. 

 In consequence of this a basal adhesive plate does not develop. The 

 whole wall of the young colony is derived exclusively from that part 

 of the body-wall which, in the larva, lined the mantle-cavity. The 

 condition of the mantle-fold consequently here resembles that in the 

 marine forms. 



-/ 



■A 



yf— 



Fig. is. — Three consecutive stages in the fixation of the larva of Plumatdla (after 

 Beaem). /, mantle-fold (in Ji and C reflected); k, buds; p, the two first 

 developed polyjndes ; ,«, glandular part at the posterior end of the body, by 

 means of which the first attachment takes place. 



/ 



IV. Development of the Polypide. 



We must now describe more in detail the development of the 

 polypide, i.e., of the retractile cephalic section of the animal plus 

 the intestinal canal appended to it. For our knowledge of the 

 metamorphosis of the larva and the development of the polypide in 

 the primary zooecium, we are indebted chiefly to the observations of 

 Repiachoff (Xo. 29) on Tendra, of Barrois (No. 9) on Lepralia, 

 and of Prouho (Xo. 28) on Fhistrella. The development of this 

 primary polypide takes place in exactly the same manner as that 

 of the polypides in the buds, which develop later in the colony, 

 or in those zooecia in which the alimentary canal and polypide, as 



