of the Fishery Board for Scotland. 



253 



acquire the brown tinge characteristic of hepatic tissue. They increase 

 rapidly in size. At first the intestine forms a simple arch from the pylonus 

 to the anus, but it soon lengthens, and, bending abruptly to the left, forms 

 a loop (PI. XH. fig. 31, i). During development it loses its early rugose 

 and granular structure, becoming translucent, especially near the anus, 

 where, when dilated, as it often is, the double contour of its walls is well 

 seen. The oesophagus is large, and leads from the posterior border of the 

 velum to the stomach by a slightly sinuous, and gradually narrowing, 

 course. It is lined by stout cilia, whose vibrations direct the currents 

 inward (PI. XII. fig. 31, oes). Its inner termination is surrounded by a 

 circlet of cilia directed into the stomach cavity. They are often agglutin- 

 ated together by mucus. The stomach-cavity is divided into two portions 

 by a well-defined band (PI. XII. fig. 31, bn). It is clothed with cilia, 

 which cause the ingested particles to gyrate. In some cases these cilia are 

 seen to vibrate in wave-like zones. 



Embryos presenting the features above indicated are "134 mm. in 

 length, and are at most twelve days old. Many reared artificially reached 

 this stage, but although kept alive for forty days they made no progress in 

 development, mostly lying closed at the bottom of the beakers they were 

 reared in. On the 4th June, by simply dipping from the surface of the 

 sea over rocky mussel ground at high tide, numbers of embryos were pro- 

 cured from the trochosphere stage up to that described above. Many 

 were kept in vessels standing in. running sea-water, but they also ceased to 

 develop. By means of tow-net and bottle attached, occasional examples 

 of more advanced forms were procured in St Andrews Bay, at various 

 points from the shore to beyond the three-mile limit. The largest ex- 

 ample seen swimming (for an instant) by means of its huge velum, 

 measured '188 mm. This specimen (PI. XIV. fig. 3) was taken while on 

 board the 4 Garland.' In the contracted state the organs are so packed 

 together as to be difficult to detect. 



The embryo, immediately previous to the deposition of the blue pris- 

 matic shell substance, is *28 mm. in longest diameter, and fairly trans- 

 lucent. At this stage the posterior adductor has become well developed 

 (PI. XII. fig. 32, pa). The liver is composed of large loosely -aggregated 

 globules. The intestine has elongated greatly, and when seen, in the 

 contracted state at all events, its condition approaches that found in the 

 adult. The velum and its retractors are still present. The oesophagus is 

 similar to that seen in earlier forms. The foot is differentiated, and 

 three or four gill-papilke have arisen posteriorly. An aggregation of dark- 

 brown particles on each side over the oesophagus is the eye-spot, and in 

 proximity to it, dorsally and posteriorly, is the auditory capsule, with its 

 contained rotating otoliths. The outlines of the shell is now roughly oval, 

 broadest posteriorly, and the hinge sunk between two lateral elevations of 

 the valves. The pallial muscle is wide and striated. The mantle is 

 ciliated internally. The oval space (PI. XII. fig. 32, v) immediately 

 anterior to the posterior adductor is identified with the organ of Bojanus 

 by Loven and Lacaze-Duthiers,* but not without doubt. 



In later forms the prismatic shell-substance begins to be deposited 

 round the margins of the valves. Its growth takes place most rapidly 

 antero-posteriorly, the mussel thus assuming the elongated form charac- 

 teristic of the adult. As it gains in length, room is afforded for the 

 developing gill-papillae. The foot is originally moved by two pairs of 

 muscles, one pair inserted in the umbonal region, and the other pair close 

 to the posterior adductor. The anterior retractors of the foot have not 

 yet appeared. They are probably derived from the posterior retractors 



* Loc. cit., ser. iv. torn. v. p. 28. ••■ ^ 



