190 ARTHUR WILLEY. 



The apertures of the secondary slits are at first extremely 

 small, and appear as dark spots, with transmitted light, in 

 which long cilia are to be seen working. 



In fig. 3 it will be noticed that a seventh secondary 

 thickening has been added behind, but it frequently does not 

 appear before the next stage. 



In the same figure there are fourteen primary slits, but the 

 fourteenth is only indicated in side view by a median de- 

 pression in the floor of the pharynx. This means that the 

 slit is in process of closure, as I found by occasional ven- 

 tral views, which were obtained through the struggling of the 

 animal when placed between slide and cover-glass. In this 

 way the larva frequently got on its back, and became fixed in 

 that position by the slight pressure of the cover-glass. 



The endostyle and club-shaped gland present the same 

 features as in the preceding stage. 



The metapleura are also in much the same condition ; but it 

 may be noticed that while in fig. 1 the right metapieur con- 

 curs with the left about the region of the ninth and tenth 

 primary slits, in fig. 3 it does so in the region of the seventh 

 primary slit, thus indicating that the closure of the atrium 

 has advanced forwards. 



A view of the left or oral aspect of the larva (figs. 22 — 24) 

 shows that in this stage the anterior extremity of the mouth 

 becomes no longer pointed. This is due to a continuation of 

 that hunching up of the antero-dorsal margin of the mouth 

 into the ciliated groove, of which we saw the first indication in 

 Stage I. It is ultimately carried to such an extent as to 

 entirely change the original shape of the mouth. Meanwhile 

 the anterior portion of the mouth sinks inwards deeper and 

 deeper tow^ards the other side of the body, so that it has to be 

 examined with a deeper focus than in Stage I. 



The upper part of the oral hood commences to form simply by 

 the continued growth downwards of the upper margin of the prae- 

 oral pit and ciliated groove ; while the lower part, in which alone 

 the buccal cirri have their origin, arises independently beneath 

 the under lip of the mouth. Figs. 22 — 24, besides showing the 



