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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



spicule (d) of the post-oral arm (c). The spicules (f) of the 

 pre-oral arms (a') are now very long, and they reach nearly 

 to the posterior end of the body. 



A great fold or lip (Fig. 72, o Z), now runs downwards 

 from the anterior end of the body towards the ventral 



Fig. 72. 



Fig. 72. — Ventral view of the fully developed pluteiis of Arbacia. 

 (Drawn from nature by Mr. H. Garman.) 

 For explanation see Fig. 73. 



*1 



