CRUSTACEA. 



389 



remarkable that the epipodite is developed on all the appendages anteriorly 

 in point of time to the outer ram us (exopodite). 



Although in Mysis there is no free larval stage, and the development 

 takes place in a maternal incubatory pouch, yet a stage may be detected 

 which clearly corresponds with the Nauplius stage of Euphausia (E. van 

 Beneden, No. 465). At this stage, in which only the three Nauplius 

 appendages are developed, the Mysis embryo is hatched. An ecdysis 

 takes place, but the Nauplius skin is not completely thrown off, and 

 remains as an envelope surrounding the larva during its later develop- 

 ment. 



Decapoda. Amongst the Decapoda the larva usually leaves the 

 egg in the Zosea form, but a remarkable exception to this general 

 rule is afforded by the case of one or more species of Penseus. Fritz 

 Miiller was the first to shew that the larva of these forms leaves the 

 egg as a typical Nauplius, and it is probable that in the succes- 

 sive larval stages of these forms the ancestral history of the Deca- 

 poda is most fully preserved 1 . 



The youngest known larva of Penseus (fig. 214) has a somewhat oval 

 unsegrnented body. There spring from it the three typical pairs of 

 Nauplius appendages. The first is uniramous, the second and third 

 are biramous, and both of them adapted for swimming, and the third 

 of them (mandiblesj is without a trace of the future blade. The body 

 has no carapace, and bears anteriorly a single median simple eye. 

 Posteriorly it is produced into two bristles. 



After the first 

 moult the larva has a 

 rudiment of a forked 

 tail, while a dorsal 

 fold of skin indicates 

 the commencement of 

 the cephalo-thoracic 

 shield. A large pro- 

 visional helmet-shap- 

 ed upper lip like that 

 in Phyllopods has also 

 appeared. Behind the 

 appendages already 

 formed there are 

 stump-like rudiments 

 of the four succeeding 

 pairs (two pairs of 

 maxillae and two pairs 

 of maxillipeds) ; and 

 in a slightly older larva 

 the formation of the 

 maudibular blade has 



FIG. 214. NAUPLIUS STAGE OF PEN^SUS. 

 (After Fritz Miiller.) 



1 The doubts which have been thrown upon Miiller's observations appear to be 

 iRiite unfounded. 



