28 



PART I. GENERAL ACCOUNT 



Though useful as a specific distinction, the number of tentacles is by no 

 means invariable within a given species. Thus in adult, sexually mature 

 individuals of Polybrachia annulata the number varies between 26 and 39, 



cau 



Fig. 18. Tentacular crown of Spirobrachia grandis. Viewed from the ventral side; ventral tentacles 

 cut away. 



cav - intertentacular cavity ; lof - lophophore ; te - outermost tentacle ; ti - innermost tentacle. 

 (After Ivanov, 1960a.) 



and in Spirobrachia beklemischevi from 39 to 72. Detailed investigation has 

 shown that in adult Lamellisabella zachsi the number of tentacles gradually 

 increases by the growth of a very few new tentacles, whose formation is 

 concentrated at the ventral ends of the horseshoe of the crown. In Spiro- 

 brachia a particularly active zone of formation of new tentacles is located at 



