136 



Part III. — Twenty-first A^inual Reimt 



III.— ON THE LARVAL AND EARLY YOUNG STAGES, AND 

 RATE OF GROWTH, OF THE SHORE-CRAB {CARCINUS 

 MJiJNAS), Leach. By H. Ohas. Williamson, M.A., D.Sc, Marine 

 Laboratory, Bay of Nigg, Aberdeen. (Plates vii.-xiii.) 



CONTENTS. 



PAGE. 



Introduction, . . . . . . ,136 



The Hatching Period : the Occurrence of the Zoea, , . 137 



The Larval Stages, ...... 139 



The Larval Characters, . . . . .140 



Development of the Appendages : — 



Antennule, ...... 142 



Antenna, . ..... 144 



Mandible, . . . . . . 145 



First Maxilla, ..... . 145 



Second Maxilla, ...... 146 



Maxillipedes, . . . . . .147 



Third Maxillipede, and Pereiopods, . . .150 



Branchiae, ...... 155 



Abdomen, Telson, Pleopods, .... 156 



External Sexual Characters, . . . .160 



The Eye, . . . . . . 160 



Carapace, . . . . . . . 161 



Thorax, ........ 162 



Sizes of the difiFerent Stages, . . . . .162 



The Rate of Growth, . . . . . • 



Literature, . . . . . . .174 



Explanation of Plates, . . . . . .175 



INTRODUCTION. 



A considerable amount of attention has at various times been devoted 

 to the larval stages of Carcinus mcenas. This crab is one of the 

 commonest on the British coasts, and its Zoea may be easily captured 

 near the shore by the tow-net during the spring and summer. 



J. Vaughan Thompson first demonstrated the fact that the Zoea and 

 Megalops were developmental stages in the life-history of the decapod 

 Crustacea. In 1828 he described* the larva of Cancer pagur us, the 

 eggs of which had hatched under his observation. The larva proved to 

 be a Zoea, "which presented exactly the appearance of Zoea taurus, 

 with the addition of lateral spines to the corselet." Later, in 1835, in 

 the papert in which he showed that the Megalops is also simply a young 

 stage of a crab, he described a Zoea as that of Carcinus mcenas which 

 clearly does not belong to this species. The Zoea of the latter has no 

 lateral spines on the carapace, whereas that figured by Thompson has a 

 very prominent lateral spine. This mistake was followed by Claus.J 



* J. Vaughan Thompson, Zoological Researches," vol. i., Pt. I., Cork, 1828. 

 [According to Cano, Cavolini was the first to recognise the Zoea. He described and 

 figured the larva of Pachygrapsus mar moratus {^^ M.oxio\og\-^ dell'aparecchio sesBuale 

 feminile ecc. nei Crostacei Decapodi " — Mittheil. a. d. Zool. Stat. Neapel^ ix. Bd., 

 4 Heft, 1890, p. 505.] 



t J. Vaughan Thompson, "The Double Metamorphosis in Decapod Crustacea," Phil. 

 Trans. Roy. Soc, London, 1835, p. 359, 1 pi. 



% Claus, " Zar Kenntniss der Malacoatrakenlarven," WurrMirqer Natimv. Zeitschr : 

 1861. 



