88 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



claims, is the prostomium, the other two segments being the first and 

 second true somites {I, II). (See also SoUaud, 1933.) 



The caudal papilla of the malacostracan embryo (fig. 38 D, E, 

 CdP) projects from the blastoderm. In its distal part is a circle of 

 undifferentiated cells, ectodermal (EcT) and mesodermal, which are 

 the teloblasts that will generate the postnaupliar somites. Beyond 

 the teloblasts is the region of the telson (Tel) containing the anus 

 (An). In its development the caudal papilla bends forward (F) 

 beneath the part of the embryo contained in the blastoderm. 



When the malacostracan embryo reaches the metanauplius stage 

 there appear at the base of the caudal papilla the two maxillary somites 

 and their appendages (fig. 38 F, iMx, 2Mx). In a study of the 

 development of Hemimysis, Manton (1928) includes the two maxil- 

 lary somites in the part of the body produced from the teloblasts. 

 Sollaud (1923), however, asserts that in Leander both maxillary 

 somites arise from the base of the caudal papilla before the beginning 

 of activity in the teloblast, and that the first somite of the teloblastic 

 series is that of the first maxillipeds. According to Sollaud, there- 

 fore, the four somites of the metanauplius (F), namely, those of the 

 second antennae, the mandibles, the first maxillae, and the second 

 maxillae, are primary somites formed directly in the primitive em- 

 bryonic body between the acronal prostomium and the caudal papilla. 

 If so, it would seem to be more than a coincidence that the same 

 number of primary somites occurs in Malacostraca, Xiphosurida, 

 and Trilobita. 



In most of the entomostracan Crustacea the embryo hatches in the 

 nauplius stage when only three pairs of appendages are present 

 (fig. 4B). The trunk is not yet distinctly segmented, but it con- 

 sists of three regions. The first region is a preoral cephalic lobe 

 bearing a median eye, the first antennae (lAnt), and the labrum; 

 the second carries anteriorly the second antennae {2 Ant) and the 

 mandibles (Md), and includes posteriorly the area on which the first 

 and second maxillae will be formed ; the third region is a terminal 

 unsegmented lobe, the telson, at the base of which is the generative 

 zone from which will be formed the teloblastic somites. The nauplius, 

 therefore, represents an ontogenetic stage in which the body region of 

 the four primary somites is present, though the appendages of the 

 posterior two of these somites are as yet undeveloped. 



The crustacean nauplius has often been likened to the trochophore 

 larva of the Polychaeta (fig. 4 A), and the two forms are comparable 

 in so far as each represents an early stage of ontogenetic development. 

 We cannot suppose, however, that the arthropods and the annelids 



