PACKARD.] MORPHOLOGY OF PHYLLOPODA. 379 
the whole preocular subregion is wanting, the head under the eyes rap- 
idly retreating backward and downward. This great development of 
the preocular region is probably connected with the burrowing habits 
of these erustacea, which take refuge in the soft mud at the bottom of 
ponds. 
In Apus and Lepidurus (Apodide) the head is shovel-shaped, being 
also adapted for burrowing like a Limulus in soft mud; in this family 
the preoral region is very large, but instead of being compressed, it is 
flattened vertically, or shovel-shaped. 
Jn the more highly differentiated Branchipodide the head is small in 
proportion to the rest of the body, and more completely differentiated 
or separate from the thoracie portion of the trunk, and the bulk of the 
lead is composed of the preoral region; the postoral, as seen in fig. 2, 
Plate XI, carrying the mandibles and the nearly obsolete maxille, and 
forming what appears as a single segment, a little smaller than the first 
limb-bearing segment next behind it. In this family the preocular re- 
gion of Limnetis and Hstheria is reduced to a minimum and is repre- 
sented by the small triangular frontal, inter-antennal lobe, which in 
Branchipus and Chirocephalus is subdivided into two appendages of 
various complicated shapes. There is, thus, as we ascend from Limnetis 
to Branchipus a more or less gradual differentiation and condensation 
of the head; and the head of Apus approximates in form the Estherian 
type. 
The postoral region bears the mandibles and maxille, and maxillipedes 
when present, and merges insensibly into the limb-bearing or thoracic 
region (bznosome), so that there is in the Phyllopoda only a slightly 
marked cephalothoracic region, the urosome also being but slightly dif- 
ferentiated from the bznosome. 
The urosome or abdomen.—This region, so well marked in the Deca- 
poda, is in the, lower Phyllopods not differentiated from the cephalo- 
thoracic, no arthromeres being in the Limnadiade interposed between the 
last limb-bearing or appendigerous arthromere and the telson. The ex- 
ternal genital organs, which may serve to roughly indicate the limits 
between the cephalothorax and the abdomen are wanting in the Limna- 
diade. : 
In Apus and Lepidurus the eleventh pair of feet (first pair of uropods) 
are modified to form ovisacs, but there are numerous pairs of uropoda 
beyond, and there is no regional distinction of even the slightest descrip- 
tion between the limb-bearing segments and the telson. In the Branchi- 
podide, however, a differentiation into a head, thorax, and abdomen is 
tolerably marked. As may be seen by reference to figs. 1 and 2 of Plate 
IX, and Plate XXII, fig. 3, the last pair of limbs are, in the male, modi- 
fied to form a penis- like organ, which is double at base and is developed 
from a single segment. In the female Apodide the oviducts open ex- 
ternally into the same segment as that which bears the ovisac, and we 
are disposed to regard the ovisac as an extreme modification of the 
gonopods; as in Branchipus and Artemia, Plate XXII, fig. 2, this organ 
is at base bilaterally symmetrical. The abdomen, then, of Branchinecta 
and Branchipus, for example, consists of nine segments, including the 
last, which corresponds to the telson of the lower Phyllopods. 
In the Decapoda the first pair of gonopods (there being two pairs cor- 
responding to the first and second abdominal feet of “the females) is 
situated on the first abdominal segment, and thus the Branchipodide 
somewhat approach the Decapods in this respect. It will also be re- 
membered that in Limulus the genital outlets of both sexes are in the 
first abdominal segments. Whether, however, the eleventh pair of feet 
