254 



Appendices to Fifth Annual Report 



by a portion remaining stationary, while the main mass follows the retreat- 

 ing posterior adductor — that is, as it travels backward coincident with the 

 growth of the shell. With the advent of the prismatic shell the velum is 

 atrophied, and the mussel sinks to the bottom. The foot is then used as 

 an organ of locomotion over the sea-weeds, zoophytes, &c. It is in young 

 forms capable of great extension, appearing as a long, white, highly 

 flexible thread. Its adhesive qualities enable it to be fastened at will, 

 so that by a process of full extension, adhesion, and contraction, the mussel 

 can drag itself forward with much readiness. The foot is covered with 

 uniform active cilia. The rate of growth of the mussel is very variable, 

 according to circumstances of situation, temperature, and the amount of 

 food available. On lifting, on the 2nd September, anchor, anchor-rope, and 

 buo}' (used in salmon nets), which had been submerged for four months, 

 the following measurements of young mussels were taken. On the under 

 side of the barrel-buoy mussels were found measuring 4*5 mm. and down- 

 wards, and on the anchor (in six fathoms) from 1*28 mm. downwards. 

 It cannot be doubted but that they were embryos of the current season — 

 that is, at most five or five and a half months old. The largest were found 

 close to the surface, the smallest at the bottom, and intermediate grades 

 on the rope. In such situations they occur in dense masses, more espe- 

 cially where they are intercepted in their swimming stages by the sieve- 

 like structure of feathery sea-weeds, hydroid zoophytes, and the like (PI. 

 XIV. fig. 5). 



The development of the heart is about to be investigated. As a 

 pulsating organ it appears first in young forms of *65 mm., — that is, 

 almost simultaneously with the reflection of the second lamella of the 

 inner gill. At that period the latter organ is composed of ten or eleven 

 papillae. When it has twenty papilla?, the mussel then measuring about 

 a millimetre in length, the first papillae of the outer gill-lamellae appear. 



Follicles containing developing ova have been found in mussels from 

 rock-surfaces \ inch (4 mm.) in length. It is possible that individuals a year 

 old may be sexually mature. Horst * has found the European oyster thus 

 early attains maturity. 



Sections of decalcified mussels are being studied. A striking peculiarity 

 in preparations of young forms is the tendency of the distal half of the 

 foot to bend upwards at right angles to the proximal half. Its apex lies 

 in a chamber between the oesophagus and the mantle. In a section taken 

 parallel to the foot protractor — that is, obliquely, at about an angle of 25° 

 with the long axis of the mussel — the foot is twice cut through (PI. XIII. 

 fig. l f ft). The plane of the section is represented by a line passing from 

 A to B in fig. 7, PI. XIV. The chamber containing the distal portion of the 

 foot is well defined, and bounded more or less by caeca of the liver. The 

 caeca ramify to a considerable extent in the subjacent tissue. The cells of 

 these caeca are loose and polygonal, and contain much brownish granular 

 matter in patches. The foot is bounded by columnar, ciliated epithelium, 

 and blood sinuses appear in the general muscular tissue. In the longi- 

 tudinal section of the mussel (PL XIV. fig. 7) three nerve ganglia are cut — 

 viz., the parieto-splanchnic, the supra-oesophageal (cerebral), and the 

 pedal. In the cerebral the commissure only is seen, as it passes across the 

 oesophagus. In the transverse section (PI. XIII. fig. l,pg) the pedal is well 

 seen as a large paired ganglion. They all exhibit a clearly differentiated 

 outer layer, consisting of deeply-stained, nuclei-like bodies (nerve-cells), 

 which encloses a finely striated central mass. The oesophagus in both 

 sections is seen to contain much mucus. The mucus also fills the greater 

 portion of the stomach cavity. By hardening it forms the crystalline style 

 * Horst, Verslay Oesteronderzoekingen, 1883-84. 



