140 



Part III, — Twentij-first Anrncal Report 



swimming function. In place of five there are in the later stages four 

 pairs of abdominal appendages in the female and two pairs in the male. 

 Tn both sexes these are now merely sexual organs. 



The Larval Characters. 



The Protozoea shows some yellow and brown pigment ; it is the 

 pigment of the I. Zoea which shows through the delicate cuticle. 



The Zoea of Carcinus mcenas is readily distinguished from the other 

 Zoese which may be taken along with it in a tow-net collection, made in 

 Scottish waters, by — 



(1) The absence of the lateral spine of the carapace, 



(2) The colouration. 



(3) The structure of the abdominal joints. 



On the dorsum of the carapace there is a pair of short hairs situated 

 a little behind the base of the rostral spine (Fig. 159). 



(1) The Zoese of Cancer pagurus and of certain other common 

 Brachyura have a strong lateral spine on the carapace. The carapace 

 in Carcinus is plain (ib.). 



(2) Colouration. The Zoea of Carcinus has to the naked eye a 

 greenish or olive appearance. On the dorsum just in front of the base 

 of the dorsal spine there is a double blue spot, which by reflected light is 

 brightly luminous. The pigments present throughout the body of the 

 larva are yellow and brown (or black). They are laid down in branching 

 chromatophores, and are associated, the yellow being surface, the black 

 (or brown) deeper. When examined by means of the microscope, the 

 dark pigment is seen to be arranged on the side of the cephalothorax 

 and to pass down the abdomen in a series of large chromatophores 

 located on the ventral halves of the somites. On the cephalothorax and 

 abdomen the yellow chromatophores are associated with the black. 

 Over the cornea there is a reticulation of yellow pigment. The yellow 

 pigment is more or less luminous throughout. 



In preserved Zoese the black pigment alone is seen, and the large 

 branching chromatophore on the side of the cephalothorax is charac- 

 teristic. 



(3) Abdomen (Fig. 118). The lateral part of the terga of the 2-5 

 abdominal segments extends backwards in the I. Zoea in a rounded 

 expansion over the beginning of the succeeding segment. The posterior 

 edge of this plate is minutely notched. 



I. -IV. Zoeae. All the Zoeae stages show the above characters, though 

 they vary in degree. The pigmentation varies in intensity, some forms 

 showing a much darker greenish appearance to the naked eye than 

 others. The little hairs at the base of the dorsal spine were not made 

 ou in the III. and IV. Zoese examined. 



The lateral expansion of the terga of the abdominal somites becomes 

 more prominent in the later stages ; in shape it becomes more angular 

 (Fig. 129). 



The Separation of the Four Zoea Stages. 



Zoeae of Carcinus were obtained of four different stages by means 

 of the tow-net. A certain amount of difficulty was experienced in 

 deciding that there were only four stages in the Zoea period. This was 

 due to the fact that the Zoeae of any one stage vary in size and may 

 also vary in structure. 



The Protozoea and I. and II. Zoeae were hatched out and reared in 

 the Laboratory. The III. and IV. Zoeae were captured along with I. 

 and II. stages in the tow-net. 



