36 NATURAL HISTORY OF ARTHROPODS. 



ORDER V. PHYLLOPODA. 



It is seldom that an ordinal name is more appropriate and more descriptive than 

 that applied to the group now to be discussed, for the term Phyllopoda, leaf-footed, is 

 at a o-lauce seen to be applicable in the highest degree to these beautiful and interesting 

 forms. Frequently they are considered as a sub-order of an order Branchiopoda, the 

 other divisions of which are the Ostracoda and Cladocera. 



In all the Phyllopoda except the Branchipodidse the body bears a large earapax, 

 which in the Apodida? covers the anterior portion of the body in a manner strongly 

 susi^estive of that found in the lobster and higher Crustacea, while in the Limnadi- 



ada? this armature takes the form of a bivalve shell, the two 

 halves being united by a hinge and closed by an adductor mus- 

 cle, as in the Ostracoda. This bivalve shell, into which all the 

 members can be drawn, closely resembles that of certain fresh- 

 water molluscs belonging to the genera Sphtrriion and Pist'Jiuin. 

 Morphologically this earapax is but the o-reatlv expanded dorsal 



FIG. 47 -Shell of Esthtna - ' / -,-. i 



beifragei, enlarged three portion ot the mandibular or a post-mandibular segment, and 



forms a good illustration of the hypertrophy of parts to which 



we have alluded on page 7. The two pairs of antenna? are present, but in some of 

 the Apodida? the first pair is small and the second is occasionally absent. The mouth- 

 parts are a pair of mandibles, two pairs of maxilla?, and in AJ:>HS and its allies a pair 

 of maxillipeds. The locomotive feet are foliaceous and membraneous, giving the name 

 to the order, and as portions of them (the exopodites) have respiratory functions, the 

 name Branchiopoda (gill-footed) is also appropriate. The feet vary largely in number, 

 there being fourteen in Ximnetis and sixty in some species of Apia*. There is no 

 distinction to be drawn between the thorax and abdomen, while the number of seg- 

 ments in these portions shows nearly the same variability as is exhibited by the loco- 

 motive members, there being twelve in Ximnetfs and twenty-seven in Esther ia and 

 forty-two in Aps. The abdomen terminates in a telson, which bears a pair of 

 appendages in all the genera except Thanmnocepha^us. 



The internal anatomy, as a whole, follows the usual crustacean type, and only the 

 modifications peculiar to the order need be mentioned. The heart, which occupies 

 the usual position, is a more or less elongated tube, partially divided into a series of 

 chambers by annular constrictions. The alimentary canal is a simple tube, sometimes 

 lined with glandular bodies of uncertain function. The liver is large, and, contrary 

 to the usual rule in Crustacea, is placed in the anterior portion of the body, in front 

 of the oesophagus and beginning of the stomach. The brain or supra-cesophageal 

 ganglion is very small, and is connected with the ventral chain in the usual manner. 

 Of this ventral nervous cord nothing need be said, but the brain possesses a peculiar 

 interest. In the higher Crustacea, so far as our knowledge goes, the brain is composed 

 of two or more oran^lia fused together, and from it arise the nerves supplying the eves, 



i J. * C* * 



and both pairs of antenna?, but in the Phyllopoda the brain consists of but a single 

 ganglion, and the nerves which arise from it supply only the optic organs. 



Attractive as they are in form, habit, and structure, the Phyllopoda possess a 

 higher interest when we study their development and the many curious features con- 

 nected therewith. The young Phyllopoda leave the egg in the nauplius stage, which, 

 however, presents several differences from the larva? of other Crustacea to which the 



