26o 



MEAD. 



[Vol. XIII. 



segment a pair of oar-shaped setae, and soon after one or more 

 pairs which are needle-shaped, while the body-wall is evaginated 

 to form parapodia, in which seta-muscles, etc., are developed. 

 Shortly after the dorsal setae are developed, there appear in 



all the trunk segments, except 

 the first, some little hooks, the 

 setae of the ventral parapodia 

 /^^^- (text Figs. XII-XVIII). 



Fig. XVI. — A mphitrite, 44 hours, ven- 

 tral aspect ; prob., problematic bodies ; 

 gl. mucous gland ; jnuc, subumbrellar 

 mucous gland; st., seta; i>ar., para- 

 troch; czV., cirrus. 



Fig. XVII. — Amphitriie, ventral aspect, 60 hours ; 

 ««^if.<., median tentacle; pr.b., problematic body ; 

 gl. , mucous gland ; mth. , mouth ; muse. , set«- 

 muscles; St., sets; par., paratroch; a., anus; /, 

 //, ///, /K, trunk segments. 



IV. Metamorphosis from Free-swimming to Creeping 

 Larv^. 



When the larvae have developed about five trunk segments, they 

 no longer swim about freely, but sink to the bottom and become 

 adapted to a less active mode of life, and if delicate pieces of 

 fresh ulva are put into the aquarium, they thrive much better. 



