XI 



PHYLUM ARTHROPODA 



547 



In the meantime 

 the post-mandibular 

 appendages are formed 

 in regular order from 

 before backwards : the 

 eye-stalks appear (Fig. 

 454, oc), as well as the 

 labrum (lab), and a 

 fold on each side of 

 the thorax, which is 

 the rudiment of the 

 carapace (car), and this 

 gradually extends dor- 

 sally until it meets 

 with its fellow of the 

 opposite side and 

 covers in the cephalo- 

 thorax. The embryo 

 now consists of a nearly 

 globular c e p h a 1 o - 

 thorax with a small 

 abdomen and a nearly 

 complete set of appen- 

 dages, all tucked in 

 under the cephalo- 

 thorax and closely 

 packed together within 

 the egg-membranes. 

 In this condition the 

 embryo is hatched, 

 and for some time 

 clings to the pleopods 

 of the mother or to 

 the empty egg-shells 

 by means of the pecu- 

 liarly hooked chelae of 

 its first pair of legs, 

 and is attached also 

 by a bundle of threads 

 secreted along the 

 posterior border of the 

 larval telson. 



The development of 

 the principal internal 

 organs must be re- 

 ferred to very briefly. 

 From the ectoderm 



oc 



-pre 

 at l g 



lab* 



FU. 454 Astacus fluviatilis. Two views of eggs 

 showing stages in the development of the appendages. 

 A, stage in which the rudiments of maxillae have 

 appeared and in which the abdomen has become 

 forked. B, stage in which the rudiments of thoracic 

 appendages are appearing and in which the abdomen 

 is segmented, ab. abdomen ; at 1 , antennule ; at 2 , an- 

 tenna ; atlg. antennulary ganglion ; car. fold which be- 

 comes edge of the carapace ; caud. f. forked extremity 

 of abdomen ; e.g. cerebral groove which gives rise to 

 the optic ganglion ; lab. labrum ; mn. mandible ; mx*. 

 first maxilla ; mx*. second maxilla ; mxpi. mxp*. mxp". 

 maxillipedes ; oc. eye-stalk ; op. g. optic ganglion ; pr. c. 

 protocerebrum ; ret. retinulse ; th. rudiments of thoracic 

 appendages. (From MacBride, after Reichenbach.) 



