CEPHALOCHORDA. 



355 



The preoral pit is at this stage large and well-formed. At its extreme anterior end it 

 originates as a small knob of tissue projecting into the head cavity (Fig. 84 PP.). In this an 

 invagination commences, which rapidly increases in size as it proceeds backwards. On the dorsal 

 side of the pit appears the preoral sense organ (Fig. 85 P. S.O.), a ciliated gutter which after 

 proceeding backwards for a short way ends blindly in front of a forward prolongation of the 

 pharyngeal region, the lumen of which opens directly behind the groove (Fig. 86). As shewn 

 in the figure there are two of these blind caeca running up from the pharyngeal region, one 

 dorsal and one ventral to the preoral pit; their walls at the ends are glandular. 



The club-shaped gland begins a little in front of the larval mouth opening. It is similar 

 in shape to that described for A. lanceolatus, and has the opening in the dorsal arm— by 

 which its cavity is put in connection with that of the pharynx— well-marked. No opening, 

 however, could be found in the lower arm connecting it with the exterior as described by 

 WiUey'. In this early stage the endostyle is not very far advanced. It shows only as a patch 

 of modified epithelium lying immediately in front of the club-shaped gland. 



Fio. 84. 



■■ P. SO. 

 PP. 



Fig. 83. 



FiQ. 85. 



The preoral Bense organ 



Fig. 83. Larva. Transverse section passing through the eye. 

 Fig. 84. Larva. Section through the anterior part of the preoral pit (PF). 



Fig. 85. Larva. Section through the middle and largest part of the preoral pit (PP). 



(P.S.O.) lies dorsally and to the left of the notochord. 



The mouth is a small elongated slit lying on the left side about one-third of the way up 

 from the ventral side. The obscure organ termed Hatschek's nephridium lies on the left side 

 just under the notochord and runs into the pharynx. 



The first gill slit opens under the mouth and a little to the left of the mid-ventral line 

 (Fig. 87 G.S.I). The next two slits also lie somewhat to the same side gradually approaching 

 the mid-ventral line, in which the rest are placed. The right metapleural fold arises at a more 

 forward level than the left, which starts from the hind corner of the mouth between the 

 second and third gill slits. It rapidly increases in size until it equals its fellow of the right 

 side. Between these folds the gill slits occupy a mid-ventral position. 



' Willey, loc. cit. 



G. 46 



