10 MORPHOLOGY OP 



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 afiirmed as to the course 

 of the blood. As will be seen in considering the internal 

 structure, a large trunk appears (2 — 3 ff. s.) 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 amceboid-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 



