REVIEW OF SPENGEL’S MONOGRAPH ON BALANOGLOSSUS. 411 
finally decisive against an assumption of chordate affinities, 
How little weight is really to be attached to it in 
the case of the Protochordata can be seen by con- 
sidering the case of Tunicata, where the direction of 
the blood-flow is periodically reversed. This fact seems to 
have escaped Professor Spengel’s memory. 
The attempt to prove that the gill-slits of Balanoglossus and 
Amphioxus, in spite of their extraordinary similarity, are 
morphologically different structures, is exceedingly weak. The 
facts adduced against their homology in the two animals, viz. 
the more dorsal position of the gill-pores in the Enteropneusta— 
the presence of ccelom in the tongue-bar and its absence in the 
primary branchial septum in the one case, and the opposite 
arrangement in the other—the slightly different arrangements 
of the gill-skeleton, are all points of detail. 
As a matter of fact, the gillsof Balanoglossus are more 
typically Vertebrate in structure than those of 
Amphioxus, inasmuch as in the former case, as in 
all the higher Vertebrata, we have gill-pouches, 
whereas in Amphioxus we have merely slits. These 
latter obviously correspond not to the small ex- 
ternal gill-pores, but to the internal gill-slits of 
Balanoglossus, the outer portions of the gill-sacs 
having atrophied in Amphioxus in consequence 
of the development of the atrial fold, just as in 
Teleostei the development of the operculum has 
hadasimilar effect. It is true that in many of the species 
of Enteropneusta these inner slits do not as nearly reach the 
middle line as they do in Amphioxus, but we trust no serious 
morphologist will ask us to consider this an important 
difference, more especially as in Schizocardium they nearly 
meet in the middle line. 
The fundamental plan of the gill-skeleton is the same in 
Enteropneusta as in Amphioxus, as it may be considered to 
consist in both cases of a series of U-shaped rods. These, in 
the latter case, have all united to form a continuous lattice- 
work ; the persistence of coelom in the tongue-bars of 
