THE CRUSTACEA 23 



cephalic (naupliar) region probably arises in connection with the 

 lips of the blastopore and consists of mesenchymatous cells. In 

 the region of the trunk, in many cases, paired mesoblastic bands 

 are formed, growing in length by the division of teloblastic cells at 

 the posterior end and becoming segmented into somites. The 

 occurrence of true coelom-sacs is doubtful. The rudiments of the 

 first three pairs of appendages, antennules, antennae, and mandibles, 

 commonly appear simultaneously and, even in forms with embryonic 

 development, they often show differences in their mode of appear- 

 ance from the succeeding limbs. When this stage, corresponding 

 to the nauplius stage of larval development, is passed through 

 within the egg, it is often marked by the formation of a cuticular 

 membrane within which the further development proceeds. 



The complex and varied larval metamorphoses of the Crustacea 

 have been the subject of much discussion in view of their bearing 

 on the phylogenetic history of the group. In those Crustacea in 

 which the series of larval stages is most complete the starting-point 

 is the form already mentioned under the name of nauplius. The 

 typical nauplius (Fig. 6, p. 11) has an oval unsegmented body and 

 three pairs of limbs, corresponding to the antennules, antennae, 

 and mandibles of the adult. The antennules are uniramous, the 

 others biramous, and all three pairs are used in swimming. The 

 antennae may have a spiniform or hooked masticatory process at 

 the base, and share with the mandibles, which have a similar 

 process, the function of seizing and masticating the food. The 

 mouth is overhung by a large labrum or upper lip, and the integu- 

 ment of the dorsal surface of the body forms a more or less definite 

 dorsal shield. The paired eyes are, as yet, wanting, but the median 

 eye is large and conspicuous. A pair of papillae or filaments, 

 probably sensory, are commonly present at the anterior end. 

 Nauplius larvae, differing only in details from the typical form 

 just described, are found in very diverse groups of the Crustacea, 

 such as the Branchiopoda, Copepoda, Cirripedia, and some Mala- 

 costraca. In many forms which hatch from the egg at a more 

 advanced stage there is, as already mentioned, more or less clear 

 evidence of an embryonic nauplius stage. It seems certain, there- 

 fore, that the possession of a nauplius larva must be regarded as 

 a very primitive character of the Crustacean stock. 



As development proceeds, the body of the nauplius elongates 

 and indications of segmentation begin to appear in its posterior 

 part. At successive moults the somites increase in number by the 

 addition of new somites, behind those already differentiated, from 

 a formative zone in front of the telsonic region. In the most 

 primitive cases, the appendages posterior to the mandibles make 

 their appearance, like the somites which bear them, in regular 



