1881.] Variations in a Copepod Crustacean. 691 
formed pair of legs, and finally the abdomen with the terminal 
furca. Viewing the remaining “carcass” from which those ap- 
pendages have been taken, we will notice that there is a certain 
demarcation between head and thorax, forming a segment ; fol- 
lowing this we find five more thoracic segments, of which the 
fifth is half as long as either the second or third, the fourth slightly 
shorter than the second or third, which latter two are equally long, 
the first being somewhat longer. 
In the female the first and fourth thoracic segments are longer 
than the second and third, the fifth is faintly sub-segmented on the 
dorsal side, laterally terminating in a strong spine similar to /chthy- 
ophorba denticornis Claus (Opus citatum, p. 199, Tab. xxxv, Fig. 1). 
We now place the glass slide under a compound microscope, 
applying a low magnifying power and inspect the left, normally 
shaped, anterior antenna (see Fig. 1). 
It is beset with pretty large bristles, and consists of twenty-five 
joints. When connected with the body, the fifteenth joint will 
reach to the base of the abdomen, and the terminal bristles reach 
to near the tip of the furcal bristles. The anterior antenne 
originate from the first pair of larval legs, and are the means with 
which the Diaptomus performs its peculiar jerking motions, de- 
: scribed by Herrick in AMERICAN NATURALIST, 1879, page 622. 
In glancing at the right antenna of 
the male (see Fig. 2) one would 
think it had been taken from an 
entirely different species—so dissimi- 
lar it looks! If we count the joints, 
we find but twenty-three, two joints 
less than in the left antenna. But 
cither the tenth or twelfth joint must 
Consist of two connate joints, and the 
twenty-first is evidently also sub-seg- 
mented, making in all, twenty-five 
joints. The sixteenth to the nine- 
teenth joints, inclusive, are consider- 
ably dilated and swollen, enclosing a 
Powerful muscle, inserted near the 
fifteenth and in the twentieth joint; 
thence follows the knee- shaped sec- 
tion of the antenna, the geniculatin 
Fic. 2. Bent anterior an- 
tenna of m 
bs Part, consisting of a larger joint with an inner + duplicatre or 
