MUSEUM OF COMPAEATIVE ZOOLOGY. 203 



EXPLANATIO]^" or THE PLATES. 



PLATE I. 



Prionospio tenuis Veeb. 



a r. Anterior region of the body. 



ag. Anal gland. 



bt. Basal joint of the tentacle, wliich, as the larva grows older, increases its size 



at the expense of the embryonic tentacle (t). 



c h. "Crochet hook " spines. 



ff. Glandular body which originates as a sac-like prolongation of the stomach 



cavity, becomes pigmented, and is ultimately lost in older larvse. 



h. Heart. 



m. Mouth. 



m s. Cluster of pigment spots found on the fourth body segment. 



o. Ocellus. 



/'. Proboscis. 



p l. Praeoral lobe. 



p r. Posterior region of the body. 



s. Embryonic setae. 



Figs. 1, 2. Young larvas of P. tenuis. 

 " 3. Young of the same, a little more advanced in age. 

 " 4. Head of the last larva from the ventral side. 

 " 5. Still older larva in which an articulation has appeared at the base of the 



cephalic tentacle. 

 " 6. The same, with the cephalic sets pressed to the sides of the body. 

 " 7. Still older larva. 

 " 8. Side view of the last. 

 " 9. Ventral view of the head of the same. 

 " 10. Side view of an older larva. 



11. Older larva, in which the cephalic spines have disappeared (dorsal view). 



12. More developed larva in which the provisional cephalic tentacles have 



been replaced by the permanent branchiae. 

 " 12 . Posterior segment of the last larva (dorsal view). 

 " 13. Oldest observed larva of P. tenuis. 

 " 18. " Crocliet hook" spines of posterior segments. 

 " IS*". One of the same magnified. 



