THE ENTBROPNEUSTA. 47 



cells lining these folds are similar to those of the digestive 

 tract, containing large granules and fluid-looking vacuoles. 



The skin covering these liver-saccules is very thin, and in 

 B. salmoneus it may often be seen fused with the 

 hypoblast, forming openings which place the cavity 

 of the liver diverticula into actual connection with 

 the exterior. The histological appearances are such as to 

 leave no doubt that an actual fusion occurs. When the 

 extreme softness of the tissue is remembered, it seems likely 

 that these perforations may, in the first instance, be due to 

 wounds which have healed so as to form fistulse. [In a single 

 case of B. minutus a fistula of this kind was found forming 

 a perforation from the intestine to the body cavity. In this 

 animal the fusion between hypoblast and mesoblast was quite 

 complete.] 



The liver ofB. salmoneus is dark green in colour, and this 

 colour is due to minute round granules or drops in the hypo- 

 blast. In B. Eobinii the tint is generally dark brown. 



The histology of the intestine, which is usually more or 

 less diamond-shape, two of the angles being dorsal and ventral, 

 is in no way remarkable. From the first the wall is formed 

 of a single layer of cells, ciliated, and smaller than those of 

 the digestive region {v. fig. 83). The anus opens imme- 

 diately above the tail until this structure disappears, and then 

 it opens widely in a terminal position (v, figs. 83 and 6). 



The Tail and Anal Lappets. 



The tail is present in the period between one and eight pairs 

 of gill-slits. Its skin is full of unicellular glands. The third 

 pair of body cavities are prolonged into it, and the mesentery 

 between them remains. The anal lappets (fig. 3, a) also dis- 

 appear with the tail. 



