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 Crustacea, which take refuge in the soft mud at the bpttom of 

 Ijonds. 



In A2ms and Lepidiirus {Apodidce) the head is shovel-shaped, being 

 also adapted for burrowing like a Limulus in soft mud; in this family 

 the preoral region is very farge, but instead of being compressed, it is 

 flattened vertically, or shovel-shaped. 



In the more highly differentiated BrancMpodidce the head is small in 

 proportion to the rest of the body, and more completely differentiated 

 or separate from the thoracic portion of the trunk, and the bulk of the 

 head is composed of the preoral region; the postoral, as seen in fig. 2, 

 Plate XI, carrying the mandibles and the nearly obsolete maxillae, 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 Estheria is reduced to a minimum and is repre- 

 sented by the small triangular frontal, inter-antennal lobe, which in 

 BrancMpus and Ghirocephalus is subdivided into two appendages of 

 various complicated shapes. There is, thus, as we ascend from Limnetis 

 to BrancJiipus a more or less gradual differentiatiqn and condensation 

 of the head; and the head of Apus approximates in form the Estherian 

 type. 



The postoral region bears the mandibles and maxillse, and maxillipedes 

 when present, and merges insensibly into the limb-bearing or thoracic 

 region (bsenosome), so that there is in the Phyllopoda only a slightly- 

 marked cephalothoracic region, the urosome also being but slightly dif- 

 ferentiated from the bsenosome. 



The urosome or abdomen. — This region, so well marked in the Beca- 

 poda^ is in the, lower Phyllopods not differentiated from the cephalo- 

 thoracic, no arthromeres being in the Limnadiadce 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 cejjhalothorax and the abdomen are wanting in the Limna- 

 diadw. 



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- 

 IJodidcc, however, a differentiation into a head, thorax, and abdomen is 

 tolerably marked. As may be seen by reference to figs. 1 and 2 of Plat€ 

 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 «ingle segment. In the female Apodidce the oviducts oi^en 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 Becapoda 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 Branchipodid?e 

 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 i)air of feet 



