INTRODUCTION. XXX1X 
pereion, whilst another is imserted beneath the posterior 
margin of the fourth or fifth segment. Grasping the 
female in this way, the male draws it into immediate 
contact with itself, so that the dorsal surface of the female 
presses against the ventral surface of the male. In this 
attitude, more or less firmly compressed, they swim about 
or rest on any convenient surface for many days. If the 
two be driven asunder through fear of any danger, the 
female seeks a place of shelter, while the male swims 
more actively about. Should the male swim within some 
little distance of its late companion, it becomes imme- 
diately aware of the circumstance; and we have seen it, 
after having passed the spot, abruptly turn back, seek her 
out, and seize her with avidity from amidst a numerous 
mass of others. Immediately after securing, he strikes 
her with two or three strong lashes of his tail. The 
female, rolling herself closely up, is carried off by her 
more powerful mate. 
This contact between the two sexes is either occa- 
sionally repeated, or it may last 
throughout the entire period of incu- 
bation. We have frequently taken 
them so coupled, even when the young 
have been so far developed as to be 
enabled to leave the care of the 
parent. We are induced, from this 
fact, to believe that a series of broods 
may take place successively through 
the year, and that the erotic state of 
the female may exist during the period 
of incubation. 
The penis (Fig. 15) is a soft membranous tube, that 
terminates in a small orifice. It probably has, under 
certain conditions, the power of becoming harder, but 

