VIII 



AKTHKOPODA 



203 



mxp 1 



-car 



Nauplii, showing, however, the ridge-like rudiments of two or three 

 pairs of postoral appendages. 



In the case of both Euphausia and Penaeus a succession of moults 

 leads, as in other Nauplii, to the development of the thoracico- 

 abdominal rudiment, and to the 

 appearance in it of ring- like 

 segments which first appear in the 

 anterior or thoracic portion. The 

 appendages belonging to these 

 segments, which are the maxilli- 

 pedes, are strongly developed, 

 especially the first, which has a 

 long exopodite used in swimming. 

 A cephalic shield or carapace 

 makes its appearance as a frill or 

 fold round the head region, under- 

 neath which the future paired 

 eyes appear as dark areas. In 

 Penaeus the anterior part of this 

 shield is produced into a sharp 

 rostral spine. 



The larva is now in the stage 

 known as the Protozoaea (Fig. 

 150). Its further development 

 into what is known as a Zoaea 

 larva involves the growth and 

 segmentation of the abdominal 

 portion of the body, the thoracic 

 segments remaining extremely 

 narrow, especially the posterior 

 ones, and the paired eyes become 

 stalked. In Euphausiadacea the stalks are so short that the paired 

 eyes do not project beyond the edge of the carapace, and the larva 

 is consequently known as a Calyptopis. The majority of Decapoda 

 pass through the Nauplius stage during their embryonic life and 

 only enter on their larval life as Zoaeae. 



FIG. 150. The Protozoaea larva of 

 Nyctiphanes australis. (After Spence- 

 Bate.) 



Letters as in preceding figure, oci, simple 

 anterior eye ; oc 2 , compound eye ; tli.s, thoracic 

 segments. 



ANCESTRAL CRUSTACEAN 



We may now pause to consider how far the Nauplius larva may 

 be regarded as representing an ancestral Crustacean form. Since 

 it occurs in all the lower groups of Crustacea with no greater 

 modifications than are found, for instance, in the different types of 

 larva amongst Echinoidea ; and since it also occurs in isolated cases 

 amongst the higher Crustacea ; and since, furthermore, an embryonic 

 stage corresponding to the Nauplius is clearly marked in the 

 development of every Crustacean egg which has been so far studied ; 



