VIII 



ARTHROPODA 



197 



en 



P 



organism. When the Nauplius is just hatched it remains still for a 

 few seconds, until its cuticle hardens and becomes strong enough to 

 resist the pull of the muscles. Then it starts on its active career, 

 swimming by a series of darts through the water, each dart being 

 caused by a synchronous back- 

 ward blow of all the appendages. 

 At each blow of the legs the 

 masticatory hooks seize any food 

 particles that they may encounter 

 and drive them into the mouth ; 

 feeding and swimming are thus 

 performed by the same move- 

 ments. 



If we compare this larva with 

 the stage in the development of 

 Astacus when a cuticle is first 

 formed, we can see that there is 

 a fundamental resemblance be- 

 tween the embryo in one case 

 and the larva in the other; the 

 difference between the two being, 

 first, that the appendages of the 

 embryo, since they are not func- 

 tional, are represented by mere 

 stumps ; and secondly, that the 

 embryo possesses cephalic lobes, 

 and the rudiments of compound 

 eyes, which are absent in the 

 larva. But the adult Copepod also has no rudiments of compound 

 eyes, and the origin and significance of the compound eye is still an 

 unsettled question. 



The little creature, as it begins to feed, grows, and, like all Arthro- 

 poda, can only grow by casting its cuticle. Just like the embryo of 

 Astacus, there are two growing regions, one at the posterior end of 

 the animal, which gives rise to a pointed prolongation of the body 

 equivalent to the thoracico-abdominal rudiment of Astacus, and one 

 immediately behind the third appendage. 



At the end of the first moult the larva passes into what has been 

 called the Metanauplius condition, when the small rudiments of the 

 two pairs of maxillae and the so-called maxillipede, or third maxilla, 

 appear behind the large third appendage. They are concealed by it, 

 and hence Glaus, in his original communication (1858), came to the 

 astounding conclusion that not only the adult mandibles but the 

 two first pairs of maxillae were derived from the division of the third 

 appendage of the Nauplius. The posterior end of the larva becomes 

 bilobed, the anus is in the bay thus formed, and just beneath the spot 

 where the excretory sacs of the gut are situated there are developed 

 a pair of stumps, which eventually form the first thoracic feet. 



an 



FIG. 144. The Nauplius larva of Cyclops 

 canthocarnoides from the ventral surface. 

 (After Clans. ) 



OH, anus ; at 1 , first antenna; <ii 2 , second antenna ; 

 c 7'> endopoclite ; exp, exoporlite ; gut-, Kiiathobase 



f 8 ^ anteima : ^.gnathobase of mandible; 



lab, labruin ; mn, mandible. 



