Manchester Memoirs, Vol. I. (1906), No. 3. 79 



portion is, however, reserved for the mesoderm of the 

 adult. In the Phylactolaemata, on the other hand, the 

 whole of this layer becomes mesoderm, and the cavity 

 which it circumscribes is shown by its subsequent 

 behaviour to be coelom. The larval organs, when present, 

 bear no relation to the corresponding organs of the adult, 

 but degenerate wholly and pass into the brown body. 

 The organs of the first polypide are formed from a bud 

 which is produced by an invagination of ectoderm covered 

 by a layer of mesoderm ; from the ectodermal layer are 

 developed not only the epidermis with the nervous 

 system but the whole of the digestive tract as well.^" 



It is obviously exceedingly hard to reconcile such 

 facts with ordinary germ-layer theories ; though an 

 attempt to save the situation has been made by showing 

 that the polypide bud arises (in the Phylactolaemata only) 

 near the original pole of immigration of the inner mass, 

 and that there is a complete gradation from those types 

 in which the inner mass gives rise to the larval gut and to 

 the mesoderm through those in which the endodermal 

 portion degenerates in an early stage, to those in which 

 there is never at any time any endodermal portion at all. 



The behaviour of the layers during the regeneration 

 of lost parts is as irregular as it is in budding. This has 

 been particularly shown to be the case in the regeneration 

 of the head and tail in various Annelids. The accounts 

 given by different observers, however, of the changes that 

 take place do not completely accord with one another. 



According to Rievel^° [OpJiryotrocha, Nats, Allolobo- 

 phora, LuDibriciis) a mass of ' granulation tissue ' of mcso- 



*" Korschelt, E. und K. Heider, " Lehrbu;h der vergl. Entwick. d. 

 wirbellosen Thiere," Jena, 1893-. , . 



6 Rievel, H., Zeitschr, w/ss. Zoo/., vol. 62, p. 289-341, 1897. 



