200 CRUSTACEA. 



the cephalo-thoracic section. The maxillary segments and the first 

 two thoracic segments, with the rudiments of the corresponding 

 thoracic limbs, can be recognised, as well as the commencement 

 of the metameric segmentation of the rest of the body (Claus, 

 Xo. 20). The gradual development of these larvae, as well as that 

 of Arte?nia, follows a course similar to that of Apus, if we bear 

 in mind the differences caused by the absence of a dorsal shield 

 and the consequent cramped position of the shell-gland, whose 

 opening was found by Claus (No. 21) on the second maxilla. 

 The course of development of the compound eyes can here be 

 followed very clearly ; these, together with the optic ganglia, 

 originate from a proliferation of hypodermis at the sides of the 

 cephalic segment (p. 168). In this hypodermal growth, a separation 

 into layers takes place, the inner layer being transformed into the 



Fig. 94.— So-called Nauplius stage of Estheria ticinenis (after G. Ficker). a', first, a", second 

 antenna ; md, mandible ; 61, upper lip. 



ganglion, while the superficial layer yields the actual compound 

 eye. A lateral outgrowth of this part of the head leads to the 

 development of the eye-stalk, which later becomes a movable, jointed 

 appendage (Claus). The method of development of the stalked 

 eyes in this form affords the most weighty objections to the view 

 that these organs are structures homologous with the limbs (p. 165). 

 Until the stage at which the full number of segments and limb- 

 rudiments is attained (first period of development), the antennae 

 and the mandibular palps retain the larval character of the 

 Metanaupliu8 stage. Only in the subsequent period of sexual 

 • liflVrentiation, in which the genital segments develop in a manner 

 characteristic of the two sexes, do the muscles of the swimming 

 Mltennte and the mandibular palps degenerate. The mandibular 

 pulps altogether disappear, while the second antennae shift to the 



