278 



University of California Fuhlications. [Zoolo<jy 



"iX^^ 



iS^i-: 



along at the rate of 15 mm. a day. This rapid locomotion is 

 accompanied by a rapid elongation of the frustule into an at- 

 tenuated filament. The elements of the structure are put upon 

 the stretch. Its ectoderm may become so thin as to be readily 

 overlooked. The cells of the endodermic axis, instead of remain- 

 ing short and broad, become relatively long and narrow. There 

 is no doubt that a considerable proportion of the entire length 

 of the filament is a direct result of the tension 

 ■■^ ■•'■ created by the locomotion of the amoeboid 



-^ end. A setting process which accompanies the 



{y^: ^vS stretching provides against a return of the 



jii^ filament to its earlier form. The morpholog- 



;;•, 0^;^:;:;; ical change, mechanically induced, has a cer- 



tain permanence. 



So rapid is the development of the fila- 

 ment, once the end is in contact with the 

 substratum, so pronounced the effect upon 

 the forms of both endoderm and ectoderm 

 cells, and so narrow and delicate the con- 

 nection with the stem, that there is little 

 doubt that a functional connection with the 

 latter is lost before the filament actually 

 l)reaks away. Both physiologically and from 

 the standpoint of the polyp, the end of the 

 filament is the important part of it. It 

 secretes a perisarcal anchoring filament. 

 Whether it do this while attached to or 

 disconnected from the stem appears to he 

 immaterial for short spaces of time. It may 

 travel forward independently for several hours. 

 In this condition, however, there is no evidence 

 that it will ever transform into a larva. 

 In this condition, however, there is no evidence that it will ever 

 transform into a larva. 



Fig. 25. Se- 

 ries of frustules 

 developing ou 

 canal of adult 

 stem ; surface 

 view. 



YII. — Metamorphosis. 

 Aside from the development of the tentacles and frustules. 

 the polyp reaches its adult condition by a transformation so 



