TAIL-KEGiUX OF POLlSL’OTLiFMA (bDELLOSTOMA) fSTOETI. 355 
lateral veno-lympliatic sinus or trunk, r. 1. s. Above in enil.>rjonic 
condition. B. V. V. Riglit ventral veno-lymphatic trunk. Posterior 
portion lioinologous to the caudal trunk of the higher fishes. ?•. v. v. 
Above in embryonic condition. S. Gan. Spinal ganglion. V. A. 
A'entral or anal fin-ray artery. Val. (i). A'alves guarding the anterior 
orifice between the ventral veno-lymphatic trunk and the caudal heart. 
val. ( 1 ). Above in the process of formation. V. Bar. Median ventral 
cartilaginous l)ar or basal fusion of the ventral or anal fin-radials. 
V. bar. Above in the process of formation. Val. F. A^alve fold ‘extend- 
ing up into the caudal heart from the sides of the orifice between the 
heart and the ventral trunk. V. L. H<e. T. A^entral or inferior longi- 
tudinal hsemal lymphatic trunk. V. B. A^entral or anal fin-radials. 
V. r. Above in the process of formation. V. S. A'entral or anal tin 
veno-lymphatic canals or sinuses, v. s. Above in embryonic condition. 
V. Sp. N. A^entral spinal nerve ramus. V. V. A^entral veno-lymphatic 
trunk. Posterior portion homologous to the caudal trunk of the higher 
fishes. V. V. Above in the process of formation. Z. Indicates posterior 
ending of the embryonic caudal heart in reconstructions as located 
by the crossing of the third spinal nerve in the heart region. 
Fig. 1. — Rep)resents a transverse section through the tail region of an 
85 mm. Polistotrema embryo; cut passing through the anterior 
region of the caudal hearts and viewed from the rear. This section 
shosvs the enormous lateral veno-lymphatic sinuses or trunks together 
with their dorsal and anal fin branches, the longitudinal dorsal and 
ventral veno-lymphatic trunks, and the caudal hearts in transverse 
section. Observe the left lateral sinus emptying into the left ventral 
trunk, and the right ventral trunk communicating with the right caudal 
heart ; its orifice being guarded by two valves opening into the heart. 
It should be mentioned that the original section showed the great 
lateral veno-lymp)hatic sinus to be completely distended by blood- 
corpuscles. X 25. 
Fig. 2. — Is from the same series as Fig. I, but 390 microns cephalad. 
Less than one half of the section is figured. This section pDasses through 
the caudal hearts as they are about to terminate in the caudal vein. 
Xote esp^ecially that the longitudinal hgemal lymp^hatic trunks have 
united to form a single p^osterior transverse stem, which passes ventrad 
and divides into anterior and right and left branches that empty 
sep^arately into the ventral veno-lymphatic trunk after encircling the 
median ventral bar. x 25. 
Fig. 3. — From same series as above. Section is taken several slides 
cep^halad of Fig. 2, and only the main longitudinal haemal vessels are 
shown in relation to each other and to the notochord. Observe the 
tendency for the longitudinal haemal lymp^hatic trunks to divide into two 
