of the Fishery Board for Scotland. 



221 



occasion an opening into the cavity — the blastopore — presented itself in- 

 distinctly. An hour or two before this period cilia appear, and they now 

 cover the greater part of the surface of the embryo, causing it to swim 

 about and rotate actively. At forty-three hours from the time of fertilisa- 

 tion two examples (of distinct series) were characterised by having, besides 

 the usual covering of minute cilia, in the one case one strong cilium at 

 least as long as the diameter of the embryo, in the other by two cilia 

 somewhat longer. In both embryos there were features indicating ap- 

 proaching differentiation of structure. Embryos have been kept alive 

 beyond the age of the above — in fact, for four days — but as consecutive 

 data of development were not secured, and occasionally other forms were 

 accidentally introduced with the fresh supplies of water, it is advisable to 

 corroborate the notes and drawings made. Failing to solve the whole 

 problem by artificial means, many searches were made for embryos in pools 

 at the mussel-beds of the Eden, in the river itself, and in tidal pools floored 

 with mussels on the coast. The season was well advanced before this was 

 done ; nevertheless it is puzzling to understand why intermediate stages 

 were not found, seeing that mussels still bearing sexual elements were 

 abundant. None were found younger than those already referred to as 

 having been got by Professor M'Intosh on the surface of St Andrews Bay. 

 In material dipped from the surface, off the mouth of the Firth of Forth, 

 on August 29, this year, the writer found many young mussels. The very 

 youngest of them had a velum or ciliated swimming organ, which could be 

 retracted within the delicate and transparent shell-valves with great 

 alacrity. The hinge-line was long and straight. The embryo mussel is at 

 this stage many times smaller than that figured by Lacaze-Duthiers, as 

 bearing the four gill-lamellse. In more advanced forms the velum had 

 become useless as a swimming organ, the oesophagus or passage into the 

 alimentary canal was seen as a wide tract thickly clothed with strong, 

 active cilia, and the rudimentary eye -spots, liver, kidneys (organ of 

 Bojanus) and adductor muscles of the shell could also be observed. In 

 still more advanced examples the gill-lamellse appear, and the umbones or 

 beaks become elevated, so that the valves on lateral view are oval or nearly 

 circular. Still further advanced forms exhibit a foot, otoliths rotating in 

 the auditory capsule (a rudimentary organ of hearing), and many more 

 closely set gill-lamellae, while the other organs are correspondingly 

 developed. By-and-by the young mussels sink to the bottom, and 

 acquire round the margin of the embryonic shell a calcareous deposition of 

 a prismatic structure, forming a blue rim, which extends most rapidly in an 

 antero-posterior direction ; and they thus gradually assume the shape of 

 the adult. The foot of the young forms is ciliated, adhesive, extremely 

 extensile, and is the means of awkward, but active locomotion. It is pro- 

 truded forward a distance at least equal to the length of the mussel's body, 

 is fastened by its adhesive property, and then contracted, thus dragging its 

 owner along as it lies on its side. Progression is also aided by a gliding 

 muscular movement of the foot. When a suitable position is reached the 

 young musdel fixes itself there by means of its byssus or anchor-threads. 



Amongst the smallest samples of a previous season, taken from the tidal 

 rocks, the writer found that many, not more than one-eighth of an inch in 

 length, contained iu their tissue either ova or spermatozoa, presenting no ap- 

 preciable difference from those taken from the full-grown adults. The above 

 were probably not more than one year old. The fact that mussels have 

 the power of reproduction from so early a stage leads one to think of their 

 enormous fecundity, and to consider the practical aspects of the investiga- 

 tion. Great tracts of tidal rock-surface around our coasts are black with 

 the carpeting of young mussels, but these being densely crowded, out of 



