A STUDY IN MORPHOLOGY. 
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The second antenna. 
In the egg Nauplius (fig. 24, An) this is unjointed, more than half as long as the 
body, divided into two nearly equal rami, with hairs at their tips. 
In the first free Nauplius stage (fig. 25, An) a two-jointed basal portion carries a 
three-jointed exopodite and an eight-jointed endopodite. The appendage is nearly as 
long as the body, the two rami are about equal in length, and each has three long 
simple hairs at its tip. In the last Nauplius stage (fig. 26) the joints are obscure; 
the endopodite is longer than the exopodite; it has long hairs along its side, and 
those at the tip are plumose. In the Protozoea stages (figs. 27, 34, 35, 42, 43, and 
45, An, and fig. 36) it consists of a two-jointed basal portion (fig. 36), which carries an 
unjointed exopodite (ex) with long, slender, non-plumose terminal hairs, and an eight- 
jointed endopodite ( en) with eight long hairs arranged along its side and tip. The first 
and second joints are very short, while the other six are longer and nearly equal. 
In the first Schizopod stage (fig. 50, An) the appendage is rudimentary, its joints 
are absent, and the exopodite is almost but not quite as long as the endopodite. The 
appendage is only half as long as the first antenna. In the last Schizopod stage 
(fig. 54; An, and fig. 57) the exopodite has become a scale, which is only half as long 
as the seven-jointed flagellum which has become developed from the endopodite; the 
basal joint is simple, very large, and the appendage is as long as the first antenna. 
The flagellum now grows rapidly, and in the adult (fig. 61, An, and fig. 66, An) 
it has twenty-four joints, and is more than half as long as the body. The antennal 
gland opens into its base, and the scale is longer than the eye, and carries twelve long 
plumose hairs. 
The mandible. 
In the egg Nauplius (fig. 24, M) this is biramous, unjointed, and tipped with hairs. 
In the first free Nauplius (fig. 25, M) it is short, and made up of a stout basal joint; a 
two-jointed exopodite with three long slender hairs, two of which are carried by the 
terminal and one by the proximal joint; and a shorter endopodite with three long 
simple hairs. In the last Nauplius stage (fig. 26, M) the joints of the exopodite have 
disappeared, the three hairs on the endopodite have lengthened and become plumose, 
and the inner edge of the basal joint carries a hook or blade. From the beginning of 
the Protozoea series to maturity the mandible is a cutting blade, with no trace of a 
palpus, and the number of its denticles gradually increases with age. 
The metastoma. 
The manner in which the metastoma originates in the Nauplius as a pair of buds 
similar to those which become the maxillae, as well as the fact that it persists in 
closely-allied forms as a pair of limb-like structures, seems to show, as Claus has 
