THE ARTHROPODA 107 



ment. Passing over the details of segmentation and early 

 development, which indeed are not very accurately known in 

 Apus, we may proceed to the consideration of the develop- 

 ment of the larval form into the adult. The eggs, as has 

 been said, lie in the mud of dried-up pools, and on the 

 refilling of the pools larval forms of the character depicted 

 in fig. 23, A and B, are hatched out from them. This larval 

 form is known as the Nauplius. It has an oval body, the 

 anterior two-thirds covered by a dorsal shield. The mouth 

 is ventral, and is overhung by a large upper lip or labrum. 

 The anus is posterior and terminal. Anteriorly is a single 

 median eyespot which survives as the unpaired eye of the 

 adult. There are three pairs of appendages, one pair pre-oral 

 and two pairs post-oral. The pre-oral pair are simple un- 

 jointed filiform appendages bearing two terminal set£e ; they 

 become the first antenna of the adult. The second pair of 

 appendages are large biramose swimming limbs. They spring 

 from the sides of the mouth, and each consists of two basal 

 segments, of which the proximal bears a stout masticatory hook 

 directed towards the mouth, and the distal bears two setose 

 branches, an outer " exopodite " and an inner five-jointed 

 " endopodite," probably representatives of the fifth and sixth 

 endites of the typical limb of an adult. It is this pair of 

 appendages, so preponderant in size in the larva, that becomes 

 the minute and rudimentary second pair of antenna in the 

 adult. The third pair of appendages of the larva resembles 

 the second, but is smaller, and shows only a trace of a masti- 

 catory process. It gives rises to the mandibles of the adult. 

 The body behind the third pair of appendages is divided by 

 transverse constrictions into five segments in the larva of 

 Apus, but these segments as yet show scarcely any trace of 

 limbs. On the dorsal shield behind the median eye is a 

 relatively large oval prominence; the neck-organ, ^vhich persists 

 as the neck-gland of the adult anjmal. 



The Nauplius larva is of common oc'purrence among 

 Crustacea, and is particularly characteristic of the ord/er 

 Copepoda. It is only exceptionally that the posterior part 

 of the body is segmented as in Apus, in most nauplii there 

 is no trace of such segmentation. For this reason the larva 

 of Apus is often called a metanauplius. 



At the time of hatching the larva of Apus is so opaque 



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