120 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



EubrancJiipus (A). In the Cladocera (D) the general head structure 

 and composition is the same as in Aptis, except for the lateral com- 

 pression of the maxillary shield, which gives the latter its "bivalved" 

 form, but the intersegmental lines are lost, and the limits of the proto- 

 cephalon (Prtc) are marked only by the attachments of the antennal 

 muscles. The Leptostraca (F) have the cladoceran type of head and 

 bivalved maxillary shield, but are distinguished by the presence of a 

 large frontal lobe (s) projecting above the bases of the eye stalks. 

 Here again the protocephalic area of the composite head is marked 

 only by the origins of the muscles of the protocephalic appendages 

 (G), including those of the eye stalks and the two pairs of antennae. 

 The mandibles of the Leptostraca (Nebalia) retain the palpi (H, Pip), 

 but their basal structure and musculature is the same as those of 

 Euhranchipus (E) and other Entomostraca. 



The Malacostraca, other than Amphipoda and Isopoda, are com- 

 monly said to have a "cephalothorax," which includes the gnathal 

 somites and a number of following somites up to a maximum of 12 

 in all. Most of this composite structure, however, which in its fullest 

 development is covered by the carapace (fig. 51 C, Cp), is more truly 

 a gnathothorax, since the true head is always a distinct though small 

 protocephalic lobe more or less concealed beneath the overhanging 

 rostrum (r) of the mandibular somite. When the protocephalon is 

 detached, as shown in the figure (C), it is seen to be a distinct cephalic 

 structure bearing the stalked eyes, both pairs of antennae, and the 

 labrum. The typical malacostracan head is thus identical with the 

 protocephalic head of the Anostraca (fig. 50 A, Prtc). Even in the 

 Brachyura (fig. 51 D, E) the protocephalon is readily identified as 

 such, though dorsally (D) it is much reduced, and is concealed in a 

 pocket beneath the anterior margin of the carapace; ventrally (E) 

 it carries a large epistomal plate and a small labrum. In the Stoma- 

 topoda, on the other hand, the protocephalon is highly developed (fig. 

 51 F, G), and its integumental sclerotization is broken up into several 

 distinct plates {d, e, f, g), which, however, can in no sense be re- 

 garded as representing a "segmentation" of the head. The mandibles 

 of the more generalized type found in the Malacostraca (B) are 

 identical in their structure and musculature with the mandibles of 

 Entomostraca (fig. 50 E, H). 



The Amphipoda and the Isopoda (including Apseudidae), with 

 regard to the structure of the head, do not appear to be properly 

 classed with the rest of the Malacostraca, since the head (fig. 50 I, J) 

 is an intimate combination of the gnathal somites (II + III + IV) 

 with the protocephalon (Prtc), and thus resembles in its composition 



