NOTES ON THE GENUS MONSTRILLA. 567 
have been found swimming freely at or near the surface of the 
sea, the genus shows, in many respects, the characters of a 
parasitic Copepod. 
The cephalothorax comprises five apparent segments ; the 
first thoracic segment is fused with the head, and the two are 
enormously elongated, giving the animal a very characteristic 
appearance (fig. 1). The abdomen comprises two, three, or 
four segments in addition to the furca, the number of abdominal 
segments affording excellent specific characters. The first four 
segments of the thorax bear typical swimming feet ; the proto- 
podite is very large and provided with powerful muscles; the 
endopodite and exopodite have each three joints. The exo- 
podite bears a short external spine on the first and on the third 
joints, and is furnished with seven stout plumose sete. The 
endopodite is furnished with the same number of similar setz. 
In one species, Monstrilla helgolandica, there is a spine 
on the internal and lower angle of the protopodite. The fifth 
thoracic segment bears, in the female, a pair of smaller 
modified appendages. Hach member of the pair consists of a 
protopodite, from which arises a single-jointed exopodite bear- 
ing three strong setz ; internally there is a finger-shaped pro- 
cess, longer in some species than in others, which is the repre- 
sentative of the endopodite. This rudimentary endopodite has 
been overlooked by previous observers (fig. 10). In the male 
the fifth thoracic appendages are generally absent, but I have 
detected a minute rudiment corresponding to them in one of my 
specimens. 
Both males and females have a single pair of antennz, which 
in the former sex are modified for grasping; the second 
antennz, unless certain structures to which reference will be 
made below may be considered as rudiments of these appen- 
dages, are entirely absent. 
The first antenne of the female consist each of four joints, 
of which the last is nearly as long as the other three. Claus 
also describes four joints in Monstrilla helgolandica, 
but I am otherwise unable to reconcile his account with my 
specimens, for he describes the two last joints as being much 
