GNATHOSTOMATA. 



233 



-change of shape, and this enables us to separate the course of 

 metamorphosis into two periods, the first comprising the series of 

 Nauplius and Metanauplius forms, while the second is distinguished 

 by a name taken from the metamorphosis of the Cyclopidae as the 

 series of Cyclops-like larval forms. In the first series, the Nauplius 

 limbs show a general resemblance to the primitive form, the abdomen 

 is not yet distinctly marked off, and the furcal processes have not yet 



Fio. 111.— Larval stages of Cetochilus septentnonalis (after Grobben, from Lang's Text-book). 

 A, Nauplius. B, Metanauplius. C, older Metanauplius. I and 2, first and second antennae ; 

 3, mandible; h, maxilla; 5, 5, exopodite and endopodite of the second maxilla ( = first and 

 second maxillipedes) ; I, II, first and second pairs of thoracic limbs ; an, anus ; g, brain ; gz, 

 genital cells ; m, mouth ; me, primitive mesoderm-cells ; 61, upper lip. 



developed. In the second series, these last advances in development 

 are made, while the antennae and mouth-parts approach the adult 

 form. 



The development of the free-living Copepoda has been specially 

 investigated by Claus (Nos. 64 and 67). We take, as the foundation 

 of our description, the metamorphosis of Cetochilus, which has been 

 minutely described by Grobben (No. 73). The free-living Copepod 

 leaves the egg as a strikingly unspecialised Nauplius (Fig. Ill A). 



