140 Part II[.—Twenty-first Annual 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. 
Tue LArRvaAL CHARACTERS. 
The Prorozo#a shows some yellow and brown pigment ; it is the 
pigment of the I. Zoéa which shows through the delicate cuticle. 
The Zo#a of Carcinus menas is readily distinguished from the other 
Zoée 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 Zoée of Cancer pagurus and of certain other common 
Brachyura have a strong lateral spine on the carapace. The carapace 
in Carcinus is plain (7b.). 
(2) Colouration. The Zoéa 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 Zoée 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. Zoéa in a rounded 
expansion over the beginning of the succeeding segment. The posterior 
edge of this plate is minutely notched. 
I-IV, Zoée. All the Zoéx stages show the above characters, though 
they vary indegree. 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. Zoée 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 Zoéa Stages. 
Zoée 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 Zoéa period. This was 
due to the fact that the Zoée of any one stage vary in size and may 
also vary in structure. 
The Protozoéa and I. and II. Zoée were hatched out and reared in 
the Laboratory. The III. and IV. Zoée were captured along with I. 
and II. stages in the tow-net. 
