G. A. DREW ON YOLDIA LIMATULA, SAY. 3 



MANTLE. 

 FIGURES 1 AND 3. 



The lobes of the mantle are free along their ventral borders, and are 

 ciliated in patches on their inner surfaces. They are modified to 

 form the siphons, the marginal tentacles, an unpaired, very much 

 elongated, tentacle situated near the base of the siphons, a flattened 

 expansion opposite the extremity of each radiating stripe on the pos- 

 terior end of the shell, and a rounded projection opposite the extremity 

 of each radiating stripe on the anterior end of the shell. These will 

 be treated in turn. There are also two pairs of patches that are 

 apparently glandular. One pair is situated ventral to the anterior 

 adductor muscle, and the other ventral to the base of the siphons. 



Siphons. The oldest specimens reared from eggs did not begin 

 to develop siphons. The youngest specimen collected with a dredge 

 (about .5 mm. long) had already formed the exhalent siphon. 



This siphon, Fig. 11, seems to have been formed by the union of 

 the margins of the lobes of the mantle, followed by their growth into a 

 tube, and the withdrawal of the tube between the lobes of the mantle. 

 In withdrawing the tube, its dorsal surface, corresponding to the dorsal 

 surface of the united lobes of the mantle, is drawn in, forming a 

 complete septum. A ridge on each lobe of the mantle indicates where 

 the point of union of the margins of the mantle has been drawn 

 along its inner surface. 



The ventral surface of the base of this siphon arches dorsally, 

 Fig. 12, the ridges on the lobes of the mantle near its base thicken, and 

 finally fuse. Thus a second tube is formed lying ventral to the exhalent 

 siphon, Fig. 13. The wall separating the two siphons remains arched 

 upward for some time, but subsequent growth straightens it. Even 

 in the adult the line of fusion along the ventral side of the inhalent 

 siphon remains distinct, Fig. 14, and offers little resistance to splitting. 

 Although the inhalent siphon is formed between the lobes of the mantle, 

 at the base of the exhalent siphon, it may morphologically be considered 

 marginal in formation, as it is formed by thickenings of, and growth 

 from, ridges that seem to have been carried back from the margin. 

 The adult siphons are united along their whole length, Figs. 1 and 3, 

 and may be extended beyond the shell to a distance considerably 

 exceeding the length of the shell. Normally the inhalent siphon is 

 shorter, broader, and has thicker walls than the exhalent siphon, but 



