ON THE ANATOMY OF HISTEIOBDELLA HOMAEI. 297 



When the animals are excited they have a rapid way of 

 opening and closing the jaw teeth, but they are never seen 

 to use them to tear open the lobster ova. When suddenly 

 disturbed they sometimes secure themselves by means of 

 their jaws to the egg membranes. They possess the power 

 of protruding the jaw apparatus considerably beyond the 

 mouth orifice, and in crawling they are sometimes seen to 

 seize some object in front of them by protruding their jaws 

 in this manner, after the fashion of many Polychgets. 



Histriobdella is remarkably sensitive to any changes in 

 the sea-water. The circulation of water through the branchial 

 chamber of the lobster insures their receiving a continual 

 change of water under normal conditions. Likewise, on the 

 " berry '^ the water is kept in constant circulation round them 

 by the ceaseless motion of the lobster's swimmerets. With 

 any slight impurity of the water they fall off their hosts, and 

 are found on the bottom . of the tank in a half pai'alysed 

 condition. They are quickly killed by the addition of small 

 quantities of fresh water, and die very readily when exposed 

 to bright light. This is of interest when it is recalled that 

 Stratiodrilus is found in fresh water. 



Fertilisation takes place internally. The male drives its 

 penis through any portion of the body-wall of the female. In 

 one instance I saw a male drive its penis into the head and 

 discharge a considei-able quantity of spermatozoa. These could 

 be seen under the microscope working their way down into the 

 generative segment. In many cases the males fertilise young 

 females without eggs, and the spermatozoa apparently remain 

 in the body till the ova develop. Many females can be 

 observed carrying spermatozoa but no eggs. 



The female exercises apparently little choice in the selec- 

 tion of a site in which to deposit her eggs. On the " berry '^ 

 these are usually attached to the membranes of the lobster 

 ova, while in the branchial chamber the carapace side is the 

 one selected. They are usually deposited in gioups of four or 

 five, and this would indicate that these are all deposited at one 

 time by the female. The eggs are all of one size, and it is 



