AN UNDESCRIBED ACRANIATE. 221 



continue as flattened cavities. Anterior to the pre-oral chamber 

 only a few very small irregular median spaces remain as repre- 

 sentatives of the fin-ray spaces. As seen in Fig. 10, the most an- 

 terior of these opens into a large unsegmented, median space that 

 comes down abruptly into proximity with the brain, where it ends 

 dorsal to the pigment spot that limits the anterior end of the brain. 



As is evident from the figure this space opens into at least one 

 of a series of long lymph canals passing back dorsal to the fin-ray 

 spaces in the substance of the dorsal fin. Such canals are seen cut 

 in cross section in Figs. 1, 3, 15, 21, 22, 23. Many of these canals 

 arise still further forward from vertical, radiating canals anterior 

 to the brain, canals that differ from those in the posterior part of 

 the body (Fig. 25) chiefly in having no swollen base, or incipient 

 fin-ray space. In the anterior region there is, however, what may 

 be regarded as a representative of an undifferentiated series of fin- 

 ray spaces developed as a single unsegmented space lying immedi- 

 ately above the notochord and extending forward from the brain 

 almost to the extreme tip of the notochord. This terminal space 

 has no connection with the one dorsal to the brain and is hence cut 

 off from the series of fin-ray spaces by an interval above the pig- 

 ment spot. 



In this interval lies a minute intermediate lymph space, as shown 

 in Fig. 10. 



Anteriorly the terminal space appears constricted into partially 

 separate, minute spaces near the tip of the notochord. A transverse 

 section taken near the tip of the notochord (Fig. 11) shows this 

 terminal space and a few of the lymph canals imbedded in the smalf 

 amount of connective tissue that makes up the dorsal fin. 



Throughout the dorsal fin the lymph canals have a nearly verti- 

 cal position at the anterior and posterior ends but elsewhere become 

 more nearly horizontal. Those from the anterior region running 

 backward (Fig. 10) incline more and more and diminish in num- 

 ber (Fig. 17), while those from the posterior end turning forward 

 supply all the posterior part of the fin. 



If now the ventral median fin be traced forward from the caudal 

 process it will be found to present the same structure as the dorsal 

 fin with the important exception that it contains no fin-rays nor 

 fin-ray spaces : thus it differs from the ventral fin of other lancelets. 



