90 WILLIAM BATESON. 
the anus remains open widely, but when the animal is irritated 
it contracts its body and draws in the intestine, closing the 
anus, over which there project two flaps of the body wall. 
These flaps are very thin and transparent presenting an appear- 
ance as of a posterior vesicle (fig. 3a). 
In some specimens a curious separation between the meso- 
blast and epiblast occurs in this place, which may be due to 
re-agents or may have some significance ; this structure will be 
treated of together with the other formations in the third 
body cavity. 
In the transparent animal pulsatory contractions can be seen 
in a vesicle lying on the dorsal side of the notochord, but 
owing to the imperfect transparency of the body walls in this 
region nothing more could be definitely affirmed as to the course 
of the blood. As will be seen in considering the internal 
structure, a large trunk appears (2—3, gs.) in the dorsal 
mesentery ; pulsations could also be observed in this structure, 
which seemed to pass from behind forwards, but occasionally 
an appearance was produced as of a reversal in the direction. 
But in consideration of the fact that the ventral wall of this 
vessel is by the nature of the case adherent to the splanchno- 
pleura, while the dorsal wall was fixed in the somatopleura, 
no very certain importance can be attached to any observa- 
tions of pulsation in the dorsal vessel, since any peristalsis in 
the gut might produce this appearance. The same applies to 
the ventral vessel, which is said to be contractile in B. 
minutus (Spengel). On the whole, however, the balance of 
evidence was distinctly in favour of postero-anteror pulsa- 
tions in the dorsal vessel. No corpuscles were discovered in the 
blood which is colourless. In preserved specimens it appears 
as a homogeneous coagulum, which in specimens preserved in 
Perenyi’s fluid shows a slight tendency to granulation. 
Large ameeboid-looking cells are visible, floating about in 
the body cavities, especially in the second. 
After about seven to eight gill-slits are formed the general look 
of the animal changes, mainly owing to the fact that the walls 
of the body become more and more opaque. This seems to be 
