of the Fisher (J Board for Scotland. 79 



0 



is comparatively very large. How is the number of zoea stages to be 

 fixed 1 The most direct method is to watch a zoea 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 zoea 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 zoeae of Crangon and 

 Carcinus variation was noticed^ especially in the size of larva?, of the same 

 developmental stage. These zoeae 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 larvae, 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 zoeae 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 zoeae 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 zoea 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 zoea 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 zoea is triangular, its hind margin fringed with 

 plumose setae. No uropods are present. The uropods appear in Stage 

 III. 



In each stage there is a marked variation in size, and the large indivi- 

 duals usually 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, la. (fig. 67), but not all, 

 showed a telson differently shaped from that of the smaller. But 

 between these two, some larvae 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 zoeae having the second form of telson. 



In Stage II. (fig. 68) the outstanding difference between the larvae 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 zoea (fig. 71, pi. 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, 



