54 MOKPHOLOGY OF 



with the skin by ducts the ova are dehisced by the breaking 

 away of whole follicles^ which then disintegrate. In the 

 branchial region of B. minutus there is a general corre- 

 spondence between these ducts and the gill-slits^ as Spengel 

 has observed. 



The testes are lobed masses placed in the same situation as 

 the ovaries. The outer zone of each testicular follicle is made 

 up of spherical cells (figs. 108 and 109j a), which contain several 

 (? eight) deeply-stained dots. These cells are young spermato- 

 blasts, and the dots, which increase in size in the spermato- 

 blasts of the inner zone, are the heads of spermatozoa which 

 are finally set free into the central cavity. Here they are 

 arranged in curious strings, which wave above parallel to each 

 other in preserved specimens (fig. 108). The testes, when 

 mature, break up in B. Kowalevskii as masses, but in B. 

 Robinii they exude from the skin as a yellow slime. 



Mucus. — All the species secrete vast quantities of mucus 

 when irritated. That of B. Robinii sets to form a mass of 

 tough consistency, which collecting grains of sand forms a sort 

 of tube. In this the animal can move slightly. The body of 

 this species is very flat in the generative region, and is 

 naturally folded up dorsalwards within the tube. The mucus 

 of this form, which comes out after prolonged irritation, turns 

 to a reddish- violet colour on exposure to the air, which is 

 very characteristic. 



In B. Brooksii, Robinii, and salmoneus the sides of 

 the body are produced dorsalwards into flaps which nearly 

 meet in the branchial region, and thus cover the gill-slits and 

 dorsal nervous system. 



