MUSEUM OF COMPAEATIVE ZOOLOGY. 203 



EXPLANATION OF THE PLATES. 



PLATE I. 

 Prionospio tenuis Verb. 



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. 



g. Glandular body which originates as a sac-like prolongation of tlie stomach 



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



h. Heart. 



m. Mouth. 



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



o. Ocellus. 



p. Proboscis. 



pi. 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 whicli an articulation has appeared at the base of the 



cephalic tentacle. 

 " 6. The same, with the cephalic setae 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been replaced by the permanent branchias. 

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

 " 13. Oldest observed larva of P. tenuis. 

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

 " 13''. One of the same magnified. 



