16:^ 



Part III, — TireiiUj-first Annual Beport 



Op. Cunningham's* description of the early young stages of Cancer 

 pagurus. In this form also the carapace of the early crab stages is 

 longer than broad. 



2. (Fig. 173). The carapace is now approaching the adult shape. 

 It is slightly broader than long. The shortening of the carapace 

 results in pushing out the toothed area on either side. 



The broad frontal region still persists. 



The minute cilia are still present on the carapace. 



3. (Fig. 170). The lateral teeth are a little farther out. The hind- 

 most tooth now projects considerably beyond the level of the corner of 

 the orbit. The frontal region is still very broad, and it shows the 

 trilobed waving. 



The lateral teeth are gradually becoming of one size. 

 Tbe " needle-point " cilia are seen on the carapace. 

 The carapace is considerably broader than long. 



4. (Fig. 171). The lateral toothed border is now distinctly farther 

 forward than in the previous stage. The frontal region remains prac- 

 tically unchanged. 



The "needle-point" cilia are still present. 



In the cast measuring 4*25 mm. of Waddington's series, No. 1, p. 165, 

 the " needle-point " cilia were not made out, but little tubercles were 

 seen scattered over the dorsum of the carapace. 



Crab 5*5 mm. across (Fig. 172). In this specimen not much change 

 in shape is noticed. The lateral teeth are now nearly all of one size. 



The minute cilia were not made out in this specimen. 



Adult. — The change from the last crab to the adult condition (Fig. 

 168, male, natural size) is a gradual lessening of the breadth of the 

 frontal region, and a pushing forward of the toothed border into an 

 ant ero -lateral position. The reduction in breadth of the frontal region 

 is accompanied by the emphasising of the trilobed waving into three 

 distinct lobes ; the median projects a little in front of the other two. 



The tri-lobed frontal region develops by the gradual pushing out of 

 the central lobe (rostrum). In the series of casts of No. 1 of 

 Waddington's collection (p. 165) the change in the frontal region is 

 well shown. In the cast measuring 7'5 mm. across, the rostrum is seen 

 to project very slightly in front of the broad frontal region. The three 

 lobes are not distinct in this series until a size of 18 mm. is reached. 

 In this stage the three lobes are well marked, but the lateral lobes are 

 not so sharply cut off from the edge of the orbit as in the adult stages. 



Thorax. 



In the I. Zoea (Fig. 85) the thorax appears in two distinct parts — 

 the anterior narrow portion which bears the first and second raaxillipedes, 

 and the posterior part bearing the buds of the third maxillipede and the 

 pereiopods. Its sternal surface is oval in shape, and it may be 

 separated off, from the buds surrounding it, as an oval plate. In this stage 

 it forms a comparatively small part of the sternum of the cephalothorax. 



During the remaining Zoea stages it increases in size with the 

 growth of the buds of the appendages, but even in the TV. Zoea it does 

 not yet bear its true proportion to the rest of the body : segmentation 

 may be traced in it (Fig. 150). 



In the Megalops (Fig. 149) the part of the thorax that has developed 

 during the Zoea period, now forms the greater part of the sternum of 

 the cephalothorax. Although the abdomen is not tucked up under the 

 thorax, the latter is depressed posteriorly along the median line. 



* Cunningham, ** On the early post-larval stages of the common crab ( Qancer pagurus), 

 and on the affinity of that species with Atelecydns heterodon." Proc. Zool. Soaj. Lond., 

 Mar. 1898. 1 pi. 



