248 



Appendices to Fifth Annual Report 



of the River Eden, near'St Andrews, for instance, examples are commonly 

 dredged measuring 4f inches, and of proportionate girth. Mussels car- 

 peting tidal rocks, although sexually perfect, are seldom of commercial size. 



General as the distribution of the common mussel is, its early life his- 

 tory has been hitherto, in great measure, matter of conjecture. Lacaze- 

 Duthiers,* in his classical memoir on the development of the gills of the 

 mussel, states that he found young forms of the length of \ mm., present- 

 ing four gill-papillae, fixed to seaweeds, &c, in the beginning of the month 

 of September. He surmised that younger embryos rose to the surface by 

 means of a ciliary locomotor apparatus, but he had not actually seen this. 



Professor M'Intosh,f while prosecuting scientific research on behalf of 

 Her Majesty's Trawling Commission, noticed that the surface of St 



Andrews Bay ' swarmed with minute mussels, much younger 



* than the forms procured by Lacaze-Duthiers in the Mediterranean, 

 'as they were settling on the blades and similar structures, within 

 ' tide-marks, and which showed four branchial processes behind the foot.' 



Loven;J figures and describes a mussel 0'586 mm. in length, having 

 eight gill filaments, but as yet possessing no heart. 



During the summer of 1885, under the direction of Professor M'Intosh, 

 and at the instance of the Fishery Board for Scotland, the writer made 

 observations on the fertilisation and early stages of development of the 

 mussel. § During last summer (1886) the investigations were continued 

 under the same auspices, and the results of these are here presented. 



Before proceeding to give embryological details, reference to the gene- 

 rative organs and products may be most conveniently made now. Histori- 

 cally treated, so early as 1761, Baster|| seems to have satisfied himself as 

 to the unisexual condition of the common mussel. Poli,1F however, some 

 thirty years later, while aware that the reproductive glands occupied the 

 mantle, held the opinion that the mussel was hermaphrodite. Treviranus** 

 figures faithfully the reproductive organs in full maturity. He hazards 

 the opinion that the ova are passed by way of the stomach out of the 

 mouth, but, being unable to believe that such a mode of exit would suffice 

 for the ova in the mantle, he suggests that the two tubes (afterwards 

 found to be the genital canals) may serve for their expulsion. He did 

 not know the mussel to be unisexual. 



Lacaze-Duthier's memoire ff (Organes Genitaux des Acephales Lamelli- 

 branches) includes the common mussel. His figure of the genital glands 

 and their ramifying canals in the mantle is diagrammatic. Sabatier J| 

 evidently made his elaborate study of the anatomy of the common mussel at 

 its period of quiescence, as little mention is made of the reproductive system. 



The genital glands are gradually developed during the winter and early 

 spring months, and come to maturity in March and April. They occupy 

 both mantle lobes almost entirely, floor the pericardial region, fill the walls 

 of the lateral cavities (the cavites des jiancs of Sabatier), swell and give 

 form to the wedge-shaped abdomen, and ramify to a greater or less degree 

 over the outer walls of the liver. In the quiescent period the mantle 

 lobes are delicate and transparent, and the abdomen (that is, the median 



* An. Sc. Nat., ser. iv., torn. v. p. 19. 

 f An. Nat. Hist., Feb. 1885, p. 151. 



X Beitrage zur Kenntniss cler Eiitivicklung der Mollusca Acephala LamcllibrancJi iata, 

 Stockholm, 1879. (Translation from Kongl. Vetensk. Acad. Handl. Stockholm, 1848.) 

 § Eeport of the Fishery Board for Scotland (Append. F, No. xiv.), 1886. 

 || Opuscula subscciva, &c, Harlemi, 1759-1765. 

 *[\ Testacea utriusque Sicilian, Paris, 1791-1795. 

 ** Zeitschr. fur Physiologic, Band i. Darmstadt, 1824, tab. v. 

 ft An. Sc. Nat., ser. iv. torn. ii. 

 XX An. Sc. Nat., ser. vi. torn. 



