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MUSEUM OF COMPARATIVE ZOOLOGY. 203 
EXPLANATION. OF THE PLATES. 
PLATE I. 
Prionospio tenuis VERR. 
Anterior region of the body. 
Anal gland. 
Basal joint of the tentacle, which, as the larva grows older, increases its size 
at the expense of the embryonic tentacle (¢). 
“Crochet hook ” spines. 
Glandular body which originates as a sac-like prolongation of the stomach 
cavity, becomes pigmented, and is ultimately lost in older larve. 
Heart. 
Mouth. 
. Cluster of pigment spots found on the fourth body segment. 
Ocellus. 
Proboscis. 
Przoral lobe. 
Posterior region of the body. 
Embryonic sete. 
1, 2. Young larve 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 setz 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been replaced by the permanent branchie. 
12°. Posterior segment of the last larva (dorsal view). 
13. Oldest observed larva of P. tenuis. 
13°. “ Crochet hook” spines of posterior segments. 
13°. One of the same magnified. 
