58 W. 0. MOINTOSH. 
the central longitudinal fibres. The shortening and elongation 
of the organs is thus explained. In vertical sections (Pl. 6, 
fig. 6) many transverse fibres occur between the longitudinal, 
often crossing at right angles to the latter. Though in such 
sections the fibres appear to be characteristically transverse, 
yet in transverse sections of the basal region (PI. 6, fig. 4) they 
present a radiate aspect, passing in front and behind the lumen 
of the blood-vessel on each side. 
These radiate fibres will readily alter the calibre of the stems, 
expansion again occurring probably by the elongation of the 
longitudinal fibres and the distension caused by the blood. 
The inner margin of each blood-vessel is well defined, appa- 
rently by a special coat, while the coagulated blood occupies 
the central space. Externally the connective tissue and other 
fibres of the branchial stem are closely united with the vessel, 
so that no separation other than what has been mentioned 
exists. In longitudinal sections in which the two vessels are 
slit symmetrically, the lateral regions are occupied by the 
latter (vessels), the median by transverse and longitudinal 
fibres. The constricted region below the tip is circular in 
transverse section, with the hypodermic cells radially arranged 
around a central point, for the blood-vessels are now absent 
(reaching only to the commencement of this narrow region, as 
in Pl. 6, fig. 5), and the fibres pass into the centre of the di- 
lated terminal region almost to the tip. In the terminal part 
the large cells of the hypoderm (which give it a vesicular ap- 
pearance) are also somewhat radially arranged—often sloping 
from the central axis outward and upward in vertical section, or 
placed regularly around the central axis in horizontal sections. 
The tissue in these dilated regions is therefore much more lax 
than in the constricted region beneath, as Ehlers observed. 
He also describes a slender, circular muscular coat exter- 
nally (that is, within the hypoderm), but the preparations did 
not satisfy me on this point, though in Euphrosyne cirrata 
certain trausverse wrinkles were seen at the base in longitu- 
dinal sections. Besides, the arrangement of the radial fibres 
would indicate that the functions of such a coat are fulfilled by 
