REPRODUCTION AND DEVELOPMENT 



129 



metamorphosis. The larva (Fig. 81) never has a free swimming 

 existence but it is adapted to living in a pool of albuminous fluid 

 within the cocoon and absorbing nutriment from it. The larval 

 organs are formed during gastrulation. The micromeres of the 



Pharyngeal muscle 

 cells 



Larval 

 ectoderm 



Blastocoe 



Thoracic 

 portion 

 germina 

 band 



Protoneph- 

 r i d i u m 2 



Mouth 



Pharynx 

 Gland cells 



Head portion 

 germinal 

 and 



Protoneph- 

 ridium I 



Endoderm 

 cells 



Degenerating 

 mocromeres 



Fig. 81. Crypto-larval stage oi Erpobdella. From Grasse, 1959, 



based on Bergh. 



first quartette proliferate and spread from the animal pole down 

 to the vegetative pole covering the germinal bands and closing oflF 

 the blastopore. These cells form a delicate larval ectoderm, 

 ciliated in the mid-ventral line, and a lining to the larval pharynx. 

 The larva is also equipped with temporary pharyngeal musculature 

 and with two to four pairs of protonephridia. It has no anus and 

 the central nervous system is still rudimentary, but albumen is 

 wafted into the gut by ciliary action and is used for growth in size 

 and complexity. 



