of the Fishery Board for Scotland. 79 
is comparatively very large. How is the number of zoéa stages to be 
fixed? The most direct method is to watch a zoéa in its development 
from hatching till it reaches the megalops stage, when it changes its mode 
of life and assumes the form of the adult. In the zoéa period every 
moult ushers in a new stage. As will be shown later, however, the 
megalops is not a fixed condition ; a large amount of variation occurs in 
its structure. The development of a single example would not be 
sufficient ; a number would be necessary. In the zoée of Crangon and 
Carcinus variation was noticed, especially in the size of larvee of the same 
developmental stage. These zoée are of very small size, whereas the 
lobster larva is large. The variation, then, in the latter is of much 
greater absolute size. The variation in size and in the development of 
the appendages together result in a multiplication of forms. Causes 
which are at present unknown—they may be food, temperature, salinity 
of the sea-water, &c.—stimulate development in certain or all the 
characters in some larvee, while apparently similar conditions of environ- 
ment result in delayed development in other specimens. The method 
adopted in the present case has been to group the zoée into as many groups 
as they naturally fall into. Of these there are three. But the extent of 
variation is sufficiently large to bring into prominence three other distinct 
forms. In the first group of zoez there are two dimorphic forms, and it 
might be inferred from that fact that we had simply to deal with two 
parallel series, but that does not appear to be the case. One case at least 
occurred where a larva belonging to one series passed by a moult 
apparently into the other series. 
The zoéa stages are very readily distinguished by the stage of develop- 
ment of the pleopods. During the zoea period the pleopods develop and 
become functional for the first time in the megalops. This occurred, 
without observed exception, in the case of Crangon and Carcinus. 
In Homarus the first zoea has the rudiments of the pleopods; they 
do not project from the abdomen; they are merely paired swellings 
on the posterior part of the under-surface of the abdominal segment. In 
Stage II. the pleopods project as unjointed bifid processes. In Stage III. 
they are large two-bladed appendages. 
In the first zoéa the cornea of the eye is attached to the carapace ; the 
eye is sessile. In the second and third stages it is quite free from the 
carapace; the eye is distinctly stalked. 
The telson in the first zoéa is triangular, its hind margin fringed with 
plumose sete. No uropods are present. The uropods appear in Stage 
FIT. 
Tn each stage there is a marked variation in size, and the large indivi- 
duals usuaily show considerable divergence in structure from the small 
specimens of the same stage, in respect to the developing appendages. 
In Stage I. one or two large specimens, Ia. (fig. 67), but not all, 
showed a telson differently shaped from that of the smaller. But 
between these two, some larve showed intermediate forms of the 
telson. Then a difference in the size of the ventral swellings (pleopods) 
on the abdomen was noticed, but the more prominent swellings were not 
confined to zoée having the second form of telson. 
In Stage II. (fig. 68) the outstanding difference between the larvee was 
that of size; a dimorphic form was not noticed here. 
In Stage III. (fig. 69) difference in the size and structure of the pleopods 
is common. The dimorphic form of the third zoéa (fig. 71, pl. iv.) is 
one which, in structure, is intermediate between Stage III. and the 
megalops. 
None of these dimorphic forms have, so far as I am aware, been 
previously recorded and described. It is possible that their origin may, 
