ON DEVELOPMENT ETC. OF PnORONIS. 537 



side of tlio mouth n transverse gvoovo, wliicli was visible wlicn 

 viewed from the ventral side. I l)elieve it was to grooves of this 

 kind that Masterman assigned the ahove im^iortant significance. 

 Tn mv opinion, they are simply artificial j^rodnctions due to preser- 

 vation. 



Tlir Collar. The form of the collar as a whole may be 

 compared to a cylinder obliquely truncated at the posterior end. 

 Its posterior border is fringed with a regular row of tentacles, 

 while antei'iorly it is joined to the hood by a narrow neck. The 

 number of tentacles (larval) varies according to the different stages 

 of growth and also aecording to the type to which larva) l)elong. 

 They are most numerous in type D, most individuals of wliich 

 bear 40-48 tentacles (figs. 12 and 1()). The rudiments of the 

 adult tentacles make their appearance as bud-like ectoblastic 

 ijiickenings immediately below the base of the larval tentacdes. 

 An exception to this rule is found in the case of larva) belonging 

 to type I), in which the adult tentacles are represented by a loeal 

 ventral thickening of the wall of the larval tentacles at their 

 jn'oximal portion (see fig. 58 c/, s. t.). It is very probable that 

 the numbc]' of the larval tentacles corresponds to that of 

 the adult. In type A, at any rate, I have ascertained that the 

 full grown larva and the worm just metamorjdiosed bear the same 

 number of tentacles, namely 10. 



The trunk. This portion, whic4i is the shortest of the three 

 regions in early larval stages (fig. 10), comes with growth to 

 occupy the largest part of the larval body and assumes a long 

 cylindrical form. Its anterior boundary is the tentacular cii-cle ; 

 the posterior end is girdled with the ])erianal ciliated belt which 

 serves as the larval locomotorv or^'an. 



