208 



INVEETEBEATA 



CHAP. 



a proper morphological representation of these in the Copepodid larva. 

 If, then, it be a sound principle of science to reason from the 

 known to the unknown, we are justified in regarding the differences 

 between the known ancestor of Adheres and the larva by which it 

 is represented, as a means to deduce the unknown ancestor of all 

 Crustacea from the Nauplius larva, by which we believe that ancestor 

 to be represented. 



A similar problem confronts us when we consider the significance 

 of the typical larva of the higher Crustacea, the Zoaea. We have 

 already seen that this larval form develops out of the Nauplius larva in 

 the Euphausiadacea amongst Schizopoda, and the Penaeidea amongst 

 Decapoda. But all the Anomura and Brachyura, so far as is known, 



begin their free-swimming life 

 with the Zoaea stage, and 

 amongst the Macrura this is 

 true of the Caridea. 



All Zoaea larvae agree in 

 possessing (1) paired stalked 

 compound eyes, (2) a carapace 

 which overlaps and conceals 

 part of the thorax, and (3) a 

 segmented abdomen. All agree 

 further in possessing (4) three 

 pairs of limbs (i.e. mandibles, 

 and first and second maxillae) 

 which are thoroughly modified 

 to form jaws, (5) in having 

 the first few segments of the 

 thorax well developed, and 

 carrying forked limbs termed 

 maxillipedes on which a large 

 part of the locomotor function 

 falls ; and finally they agree 

 (6) in having the hinder segments of the thorax either entirely 

 suppressed or very thin, and without appendages or with mere 

 rudiments of appendages. 



But whilst the general features of the Zoaea may be regarded as 

 constant, its specific features vary from group to group. The Zoaea 

 of the Euphausiadacea and of the Penaeidea is characterized by 

 retaining a large forked second antenna like that of the Nauplius, 

 which assists in swimming. 



The Zoaea of Euphausiadacea has one pair only of maxillipedes 

 well developed, and it has several other well-marked peculiarities : 

 thus the last pair of abdominal appendages is developed, and the 

 border of the carapace projects considerably and conceals the short 

 eye-stalks from view when the larva is seen from above, hence, as 

 already mentioned, it is termed Calyptopis (Fig. 155). 



The Zoaea of the tribe Penaeidea, amongst Decapoda, differ from 



FIG. 155. " Oalyptopis " Zoaea of N 

 austmlis, lateral view. (After Spence-Bate.) 



car, carapace ; Jiep, liver lobules ; oc, compound eye. 



