160 



Part III. — Twenty-first Annual Report 



The Telson (Figs. 151 and 158) now functions simply as the shield of 

 the anus. It has not yet adopted the adult form ; in the latter the 

 telson is of a triangular form, with a more or less pointed apex. The 

 telson may be raised from the thorax independently of the abdomen. 

 In this stage the dorsal surface is provided with ciliated hairs, and it is 

 also covered with the minute surface cilia. 



The Pleopods (Figs. 145 and 156) have degenerated in this stage. 

 In absolute measurement, although the crab itself has increased greatly 

 in size, they are smaller than the pleopods of the Megalops. The 

 abdomen now being closely fitted into the hollowed surface of the 

 thorax, they can no longer function as swimming organs, and they have 

 not yet taken on their adult sexual form. They are devoid of setse and 

 hooks (Fig. 156) ; one or two minute hairs are seen on the exopodites. 

 The fact that the endopodite is merely a process of the basal joint, not 

 a separate branch, is well seen in this stage. 



The fifth pleopod (Fig. 156) is a small shrunken process, in which the 

 separation into two joints is not always visible. 



2. In the second young stage the pleopods are similar to those of the 

 first young stage, but smaller. 



External Sexual Characters. 



In the second young stage the external sexual characters have not 

 appeared. I have not followed the development of the pleopods farther 

 by successive stages, but the swimmerets of the female and the penes 

 of the male appear very soon after the second stage. The breadth of 

 carapace in the latter is about 2"5 mm. 



In a crab measuring 5| mm. across the greatest breadth of tlie cara- 

 pace, the female sexual characters were found. The swimmerets were 

 present in their adult form, consisting of a 1 -jointed exopodite and a 

 2-jointed endopodite (Fig. 165). Neither branch bore any hairs at all. 

 The position of the vulvae, viz. in the twelfth somite, that to which the 

 third pereiopod belongs, was indicated by little clear depressions, but 

 there were no openings. In a female 9 mm. across, the vulvse were 

 small clear circles (apparently not yet perforated) ; the swimmerets 

 resembled those of the adult female, but very few hairs were present on 

 each branch. 



In a crab measuring 5| mm. across (greatest breadth of carapace), the 

 male sexual characters were found. The two penes were present, the 

 anterior (1 p) and the posterior (2 p), Fig. 155. No trace of vulvie was 

 seen in this specimen. The tubercles on the eleventh somite were 

 prominent. A small clear area, not very distinctly made out, was 

 noticed on the coxopodite of the fifth pereiopod, in the situation occupied 

 in the adult by the external opening of the vas deferens. 



No distinction in the breadth of the abdomen which separates the 

 adult sexes was yet apparent. 



The Eye, o, 



I. (Figs. 159, 161, 163). The eye of the I. Zoea resembles that of 

 Crangon vulgaris * in that its cornea is a specialised portion of the 

 carapace. Drawings of the moulted carapace of the first Zoea are 

 shown ; oblique side view Fig. 161, and ventral view 163. The cornea 

 is labelled, o. 



The median eye was not made out, although an irregular black area 

 was seen in the region where the median eye was to be sought for. 



Op. ci*. 



