GNATHOSTOMATA. 235 



ments of limbs, viz., the second maxillae (5), from the two branches 

 of which the so-called maxillipedes of the Copepoda are derived, 

 and the first two pairs of thoracic limbs (I, II). This stage still 

 has a distinct ISTauplius appearance. The body has grown in length, 

 but still, when seen from the side, shows the characteristic ventral 

 curvature. The posterior end of the body is still without the furcal 

 processes. The two pairs of antennae have not essentially changed 

 from their condition in the former stage, except in the increased 

 number of their setae. The masticatory hooks are still present on 

 the basal joints of the second antennae. In the mandible (3) a 

 large masticatory blade can be seen projecting from the basal joint. 

 The first maxilla (4) is like a small lobed plate, while, in the second 

 maxilla (5), indications of the separation of the exopodite (so-called 

 anterior or outer maxillipede) from the endopodite (so-called posterior 

 or inner maxillipede) are to be seen. The rudiments of the two 

 anterior pairs of thoracic limbs are found as bilobed swellings (I, II). 

 At this period, several moults take place, no essential change occur- 

 ring in the shape of the larva, beyond the appearance behind the 

 second thoracic segment of the rudiment of a third. The whole 

 series ends by a moult through which the larva passes on into the 

 series of Cyclops-like stages. 



The first of these stages, which, in accordance with the accepted 

 terminology of the Copepodan metamorphoses, we shall call the first 

 Cyclops stage (more correctly named Cetochilus stage by Grobben), 

 reveals essential changes of form. The body is no longer flexed 

 ventrally, but is straight. Its most posterior region is sharply 

 marked off from the anterior portion of the body by a constriction ; 

 the furcal appendages and the rudiment of a fourth thoracic segment 

 have developed. The limbs approximate in shape to those of the 

 adult, although they have not so many joints. The first antenna 

 has passed from the short leaf- shape into the long, cylindrical, 

 oar-like form ; it stands out laterally from the body, and consists 

 of many joints. The second antenna has remained biramose, but 

 has lost its masticatory process; on the mandible, the masticatory 

 blade has greatly increased in size. The maxilla is larger and more 

 richly jointed, the maxillipedes have become transformed into large 

 prehensile organs. The two anterior pairs of thoracic limbs have 

 developed as swimming limbs; the basal segment of each already 

 consists of two joints, but the two rami are still unjointed; the 

 third pair of thoracic limbs, on the contrary, can only be recognised 

 as a pair of bilobed rudiments. 



