314 COMPARATIVE ANATOMY CHAP. 



often show more cleai'ly than the anterior maxillae the biramose 

 character ; e.g. in the Malacostraca the exopodite is almost everywhere 

 retained as a vibratile plate. 



Entomostraca. In the Pliyllopoda, the posterior maxillae are, like the anterior, 

 reduced to simple masticatory ridges. In the Cladocera they are indeed only to be 

 found in the embryo. 



The posterior maxilla; cf the Ostracoda (F, I] show very various arrangements. 

 They sometimes function almost exclusively as masticatory organs, sometimes they 

 are locomotory organs as well, sometimes only the latter. In the first case the 

 masticatory ridge is well developed, the endopodite (feeler) small and 2-jointed, 

 the exopodite (fan plate) either rudimentary (Cypris) or very strongly developed 

 (Cypridina). In the second case the endopodite is longer and many-jointed. In the 

 third case the maxilla is formed like an ordinary limb and the fan plate has dis- 

 appeared. The arrangement of the posterior maxilla; in the Copepoda is very in- 

 teresting (A, If). The endopodite and exopodite are here retained as appendages, 

 which are usually jointed. Instead, however, of their being placed on a protopodite, 

 they are inserted direct on the body, so that we might be tempted to consider them 

 as special limbs. They have been called anterior and posterior maxillipedes. In 

 the parasitic Copepoda they serve as clinging organs and end in hooks. In the 

 Argulidcc (Fig. 195, p. 291) each of the anterior maxillipedes is changed into a 

 large adhering disc. The posterior maxilla; of the Cirripedia are small, much 

 reduced, and fused together into a sort of lower lip. They are wanting in the 

 Rhizocephala. 



Leptostraca (D). The 2cl maxilke of Nebalia are biramose, with protopodite, 

 endopodite, and exopodite. The protopodite carries 3 lobate masticatory ridges. 

 The endopodite has 2 joints. The exopodite is unjointed and narrow, and is a 

 transition form between the jointed branch and the broad flat fan plate. 



Malacostraca (B, E, G, K, L). The posterior maxilhe are more easily recognised 

 as metamorphosed biramose feet than the anterior maxilla?, in that they (except in 

 the Arthrostraca) have retained besides the protopodite and the endopodite the exo- 

 podite as the so-called fan plate. The protopodite generally carries 2 masticatory 

 ridges (lacinia;), one on the proximal, the other on the distal joint. These lacinia? 

 are often divided. 



The posterior maxilla? of the Arthrostraca are very much simplified, most of all 

 in the Ampliipoda (K), where the exo- and endo-podite are wanting, and both the 

 masticatory ridges of the protopodite are simple. In the Iso2)oda the lacinia of the 

 distal joint of the protopodite is divided ; the exo- and endo-podite are wanting. In the 

 land Isopoda and the parasitic forms even the protopodite, with the masticatory 

 ridges, is more or less degenerated. 



In the Thoracostraca the posterior maxilla; of the Schizorjoda (C) show the arrange- 

 ment above described as characteristic of the Malacostraca. Both masticatory ridges 

 are divided (Thysanopus, Eurjhausia), or the proximal remains undivided (Mysis, 

 Lophogaster, SiricUa, Eucopia). In the Cumacca the endopodite (palp) is wanting, 

 and the exopodite is small. In the Stomatopoda the exopodite is wanting, but on the 

 other hand fan-like lobes are developed on the 2-jointed endopodite. The proximal 

 masticatory ridge is undivided, the distal divided. In the Decapoda (B, E) the 

 proximal as well as the distal masticatory ridge is divided ; the endopodite (palp 

 or feeler) is small and unjointed (in the larva only has it two or more joints), and 

 the exopodite is well developed in the shape of a fan plate, with a crescent-shaped 

 process directed backwards, which regulates the streaming of water in the branchial 

 cavity. 



