THE LIFE-HISTORY OF NUOULA DKLPHINODONTA. 333 



is better described as posterior than outer. The lobes, at 

 the bases of which the outer plates are formed, develop into 

 the inner plates. Viewed from the side, both sets of plates 

 are visible. The gills of Yoldia hang so that in a side view 

 the outer plates hide the inner plates, which lie directly 

 behind them (Text-fig. L). When viewing Nucula from the 

 side we see that portion of the gill that corresponds to the 

 ventral portion in Yoldia. 



The labial palps appear as patches of cilia on embryos with 

 three gill lobes (fig. 41). The outer palps soon begin to grow 



int afna 



Text-fig. L.— Adult specimen of Yoldia limatula. Represented as seen 

 from the riji;lit side. Reconstructed to show internal organs. Fully 

 grown specimens may be 6 cm. long. aa. Anterior adductor muscle. 

 afm. Anterior foot muscles, bg. Byssal gland, eg. Cerebral ganglion. 

 es. Exhalant siphon, f. Foot. g. Gill. h. Heart, int. Intestine, is. 

 Inhalant siphon. Ip. Labial palp. ot. Otocyst. pa. Posterior adductor 

 muscle, pap. Pal|) appendage, pe. Posterior expansion of the margin 

 of the mantle, pfin. Posterior foot muscle, pg. Pedal ganglion, st. 

 Siphonal tentacle, sto. Stomach, vg. Visceral ganglion. 



out as flaps (fig. 45, Zp.), and by the time that the fifth pair 

 of gill plates are formed the inner palps are present as folds. 

 The formation of the ridges on the ciliated sui'faces of the 

 outer palps begins with embryos having six pairs of gill 

 plates, and the palp appendages ai'e formed soon after 



