348 



PALAEOSTRACA. 



a specimen three miles from the coast. After the first moult, they 

 pass into a stage which is distinguished from the preceding by the 

 richer ramification of the hepatic tubes, by the closer fusion of the 

 abdominal segments, and by the lengthening of the caudal spine. 

 This stage (Fig. 161) is worthy of notice on account of its 

 resemblance to the Hemiaspid genus Prestwichia. The later stages 

 already show the characters of the adult in all respects except sexual 

 differentiation. The latter seems to develop very late (according to 

 Lockwood, in the third or fourth year). Until this occurs, the males 

 resemble the females, they then develop strong terminal claws on the 

 second pair of limbs instead of pincers. 



3. Formation of the Organs. 

 A. Nervous System and Sensory Organs. 

 The ventral chain of ganglia develops in the form of two 

 _._ ectodermal thicken- 



ings (Fig. 15G, bg) 

 on either side of 

 the median line, 

 which enclose be- 

 tween them a thin- 

 ner portion of the 

 ectoderm. In a 

 superficial view, the 

 latter appears as a 

 neural groove, al- 

 though it is not 

 actually sunk below 

 the surface. The 

 lateral halves of the 

 ventral ganglionic 

 chain become the 

 future longitudinal 

 commissures, which 

 thicken segmentary 

 to form the ganglia, 

 and become de- 

 tached from the 

 ectoderm from be- 

 fore backward. The transverse commissures seem to arise by the 

 Invagination of the enclosed median ectodermal area (neural groove), 



FlO, Kil.— Older larval stage of Lirmdus (after Watask), 

 hepatic, diverticula; la, lateral eye; ma, median eyes. 



