BY H. L. KESTEVEN. 683 



after the appearance of tlie cap, there appears, on one side, a bay 

 in its margin ; this, as further changes show, is on the ventral side ; 

 it, therefore, gives us the second axis, and is the first appearance 

 of the bilateral symmetry of the adult. 



Tliere now follows a slow dorsi-ventral compression, which pro- 

 hal)ly reaches its maximum with the nauplius. Concurrent with 

 this, there lias been a broadening of the anterior end and a narrow- 

 ing of the posterior end, so that, in some cases, the head-end be- 

 comes decidedly broader than the posterior end ; this, however, is 

 variable. 



Meanwhile, Ihe appendages have been assuming tlie forms de- 

 picted in Fig. 46. 



At first (Fig. 42) the appendages appear as thickenings of the 

 ectoderm, situated tar forward on either side of the ventral bay in 

 the ectodermal cap. Later they pass l)ack, and the two posterior 

 pairs exliibit already tlieir future biramous character (Fig. 43). 

 The eye-spots are also now apparent. 



Fig. 9 is derived from a nnitilated embryo of the stage of de- 

 velopment of Fig. 43; but which of the two pairs of ])iramous 

 appendages it represents, I liave been unable to decide. 



Tlie external features of tlie nauplius are depicted in Fig. 46. As 

 mounted in balsam and in glycerine-jelly, the little animal is of a 

 yellow colour, transparent, showing the situation of the archen- 

 teron with its large intracellular fat gobules, also the muscles, the 

 mesoderm-cells and primordial germ-cells of the posterior end. At 

 the anterior end, the eye-spots appear as small pigment-spots sur- 

 rounded by a very clear hyaline area. The granule-laden larval 

 kidneys are visible, but the brain not so. 



No external openings are discernible, mouth and anus not having 

 formed, but between the first two pairs of appendages is a curved 

 prominence, which might be interpreted as a labrum. The whole 

 nauplius is enclosed in a very delicate cuticle, the first exoskeleton. 

 which extends beyond the body proper, or, rather, encloses the body 

 loosely, as the perisarc to the copnosarc in a zoophyte. 



The appendages are three pairs, anteriorly situated. The exo- 

 skeleton is smooth, and devoid of spines, so that the appendages 



