404 COPEPODA. 



surface of the thorax. There is in the egg a Nauplius stage with three 

 appendages, and subsequently a stage with the Zosea appendages. 



The larva when it leaves the egg has the majority of its appendages 

 formed, but is still enveloped in a larval skin, and like Mysis bends its 

 abdomen towards the dorsal side. When the larva is finally hatched it 

 does not differ greatly from the adult. 



Cuiliaceae. The development of the Cumaceae takes place for the most 

 part within the egg, and has been shewn by Dohrn (No. 496) to resemble 

 in many points that of the Isopods. A dorsal organ is present, and a fold 

 is formed immediately behind this which gives to the embryo a dorsal 

 flexure. Both of these features are eminently characteristic of the Isopoda. 



The formation of the two pairs of antennae, mandibles, and two pairs 

 of maxillae and the following seven pairs of appendages takes place very 

 early. The pair of appendages behind the second maxillae assumes an 

 ambulatory form, and exhibits a Schizopod character very early, differing 

 in both these respects from the homologous appendages in the Isopoda. 

 The cephalo-thoracic shield commences to be formed when the appendages 

 are still quite rudimentary as a pair of folds in the maxillary region. The 

 eyes are formed slightly later on each side of the head, and only coalesce at 

 a subsequent period to form the peculiar median sessile eye of the adult. 



The two pairs of appendages behind the second maxillae become con- 

 verted into maxillipeds, and the exopodite of the first of them becomes 

 the main ramus, while in the externally similar second maxilliped the 

 exopodite atrophies and the endopodite alone remains. 



The larva is hatched without the last pair of thoracic limbs or the 

 abdominal appendages (which are never developed in the female), but in 

 other respects closely resembles the adult. Before hatching the dorsal 

 flexure is exchanged for a ventral one, and the larva acquires a character 

 more like that of a Decapod. 



COPEPODA. 



Natantia. The free Copepoda are undoubtedly amongst the 

 lowest forms of those Crustacea which are free or do not lead a 

 parasitic existence. Although some features of their anatomy, such 

 for instance as the frequent absence of a heart, may be put down to 

 a retrogressive development, yet, from their retention of the median 

 frontal eye of the Nauplius as the sole organ of vision 1 , their simple 

 biramous swimming legs, and other characters, they may claim to be 

 very primitive forms, which have diverged to no great extent from 

 the main line of Crustacean development. They supply a long series 

 of transitional steps from the Nauplius stage to the adult condition. 



While still within the egg-shell the embryo is divided by two 

 transverse constrictions into three segments, on which the three Nau- 

 plius appendages are developed, viz. the two pairs of antennas and the 

 mandibles. When the embryo is hatched the indication of a division 

 into segments has vanished, but the larva is in the fullest sense a 



1 The Pontellidae form an exception to this statement, in that they are provided 

 with paired lateral eyes in addition to the median one. 



