of the Fishery Board for Scotland 157 



The branchia formula of the adult Carcinus mcenas is as follows : — 







Podobranchiaj. 



Arth robranchia; 



Pleurobranchia;. 



First Maxillipede, 



Epipodite 









Second Maxillipede, 



Epipodite 



1 



1 





Third Maxillipede, 



Epipodite 



1 



2 





First Pereiopod (chela), 







2 





Second Pereiopod, 









1 



Third Pereiopod, 









1 



Abdomen, ab, Telson, T, Pleopods, pi, Plates xi. and xii. 



I. Abdomen (Figs. 118 and 131). The abdomen consists in the 

 I. Zoea of five joints and the telson. On the hind lateral edge of the 

 first joint there is a little tubercle, from the middle of the second joint 

 a conical process pi^ojects on either side. These processes have 

 probably a mechanical function, since in certain positions of the 

 abdomen they will abut on the hind border of the carapace. They will 

 tend to limit the movement of the abdomen. The hind lateral 

 border of each of the second and fifth segments projects backwards 

 over the beginning of the succeeding joint, and it is minutely notched. 

 On the dorsal hind edge of each of these segments there is a pair of 

 short plain hairs. The integument of each joint is a cylinder. 



Pleopods. The pleopods are found in this stage as long, somewhat 

 oval bodies composed of large rounded cells ; they are situated on the 

 under-surface of the abdominal segments Figs. 118 and 131). 



These bodies are seen on all the five abdominal joints. On the first 

 joint the body is hemispherical (Fig. 181). Although in structure 

 resembling the rudimentary pleopods in the other joints, it does not 

 give rise to a pleopod. Its ultimate condition was not determined. 



The pleopod buds in the I. Zoea are referred to by Spence Bate as 

 follows : — " Close observation will detect the germs of the future 

 pleopoda, upon one or two [abdominal segments], situated laterally and 

 inferiorly.' * 



Telson. The telson (Figs. 122 and 118) is in the adult the shield of 

 the anus ; in the Zoea it, in addition to protecting the end of the gut, 

 performs other functions. It is of the form usually found in 

 Brachyurus Zoea. It is deeply forked : each leg curves gently back- 

 wards, and tapers to a delicate point. On the dorsal surface of each 

 leg there are three long teeth, of which the proximal is the smallest, 

 the distal the largest, and the median in position, intermediate in size 

 between the two others. The teeth are smooth On the inside of the 

 furca, arranged on either side of the anal angle, are three stout 

 spines which are elaborately serrated. Three different sets of 

 serrations may be made out on each side of the spine (Fig. 120). The 

 proximal third is provided with large teeth, the middle third with 

 smaller teeth, and the distal third is lined to within a short distance of 

 the sharp tip with minute serrations. The inmost spine of each group 

 has also on the inner side three long cilia ; sometimes a cilium is found 

 on its outer side. The serrations on the two sides of the spines were 

 not found to be in focus at the same time ; it is therefore probable that 



* Op. at. 



