24 



of a nev;ly-attached larva. T'-" 'mo' . ''at 

 this stage io very long, ciliated over most ol' ite extent, 

 and angular at its jioGtorior end. Tl;is angular portion is 

 occupied by a well developed, tlxough simple byssus appara- 

 tus, whicli throws out a single, simple byssus thread, sev- 

 eral times as long as th.o diameter of the larva. This 

 serves to secure th.e 1m -va in the early stages of attach- 

 ment , so that, after the veliim has been lost, it may still 

 return to its mooring if it lose its footing from any 

 cause. 



The siphons are already \foll developed in the larva, 

 the ventral (i.s. figs. 1,?.,24) with ciliated sensory pa- 

 pillae, the dorSal (e.S.), a simple non-ciliated tube. The 

 gills have advanced but liotle beyond the stage figured by 

 h'atschek (5) for the viviparous larva studied by him. On 

 each side of the body there are two large gill-slits, ■■-. 

 in the gill membrane, the rudiment of a third. Tho 

 worm" larva is a typical dimyarian. Both adductor nuscles 

 are present in their usual positions, the posterior (fig. 

 24, a. p.) already larger than the anterior (:\.y.). The 

 I'ctractors of the foot at this stage are attached in the 

 unbonal region of the shell just in front of the poste- 



