CRUSTACEA. 487 



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 Crus- 

 tacean 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 

 Nauplius appendages are developed, viz. the two pairs of 

 antennae 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 typical Nauplius 2 . There are 

 slight variations in the shape of the Nauplius in different genera, 

 but its general form and character are very constant. It has 

 (fig. 229 A) an oval unsegmented body with three pairs of 

 appendages springing from the ventral surface. The anterior of 

 these (at i) is uniramous, and usually formed of three joints 

 which bear bristles on their under surface. The two posterior 



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

 with paired lateral eyes in addition to the median one. 



2 The term Nauplius was applied to the larva of Cyclops and allied organisms by 

 O. F. Muller under the impression that they were adult forms. 



