of the Fishery Board for Scotland, 



255 



in older forms. The places of origin of the pedal muscles are indicated in 

 PI. XIV. fig. 7, pp, pr. The anterior is seen (PI. XIII. fig. 1, pp) to 

 break up in the foot into the fine lamellae of the byssus. In the 

 present section there are nine lamellae on each side of the organ. Barrois * 

 finds them to be very numerous and closely set in the adult. The gills 

 have already been studied by Holman Peck,f and the structure of the 

 shell by Ehrenbaum.;}; The aggregations of dark particles on a level 

 with the cesophagus (PI. XIII. fig. 1, e) are the so-called ocular spots. 

 The development of these and other organs of interest it is the writer's 

 intention to follow out, by means of serial sections. 



The habits of the common mussel in early life may be thus briefly 

 summarised. The eggs and spermatozoa are extruded from the follicles, 

 through the genital tubes of the respective sexes, and come in contact at 

 or near the bottom of the water into which they are shed. Fertilisation 

 of a vast number of eggs ensnes, and in twenty hours, or a shorter time, 

 the ciliated embryos rise, it may be to the very surface, in warm calm 

 weather. They are moved hither and thither by currents, and by their 

 own locomotor apparatus — the cilia, and latterly the velum. The velum 

 atrophies and they sink to the bottom, or are intercepted by sea-weeds, 

 zoophytes, nets, &c. The foot at first vermiform, adhesive, very sensitive, 

 and a means of considerable progression, is developed. At the proximal 

 region of the foot the byssus-gland secretes the byssus, wherewith, when a 

 suitable site is reached, the mussel may be anchored. 



EXPLANATION OF FIGURES. 

 Plate XII. 



Fig. 1. Ripe egg, '061 mm. in diameter, enclosed by hyaline investment, and 

 diagrammatically exhibiting nucleus and nucleolus, hy, hyaline investment ; n, 

 nucleus ; no, nucleolus. 



Fig. 2. Protrusion of polar cell from fertilised egg. pc, polar cell. . 



Figs. 3, 4, 5. Different polar cells. 



Fig. 6. Effect of osmotic action in an egg, causing the expulsion of the nucleus by 

 rupture of the egg-capsule. 



Fig. 7. Abnormal or exceptional elongation of the egg after fertilisation. 



Fig. 8. Effect of endosmotic action on the fertilised egg, causing the deutoplasmic 

 granules to pass out by way of the polar cell. 



Fig. 9. Effect of reagent (eosin) on segmenting egg, displaying the nuclei, and 

 crumpling the still adherent hyaline investment. ' 



Figs. 10, 11, 12. Eggs presenting two blastomeres, fig. 11 showing lenticular areas 

 in the sulcus, ma, macromere ; mi, micromere. 



Fig. 13. Segmentation taking place within a membrane. 



Figs. 14, 15, 16, 17, 18. Serial stages in the process of segmentation. 



Figs. 19, 20, 21. Ciliated gastrulse (the cilia not indicated in figs. 20 and 21). 



Figs. 22 and 23. Trochospheres. /, flagellum. 



Fig. 24. Trochosphere, with shell-gland, sh, shell-gland. 



Fig. 25. Velum of the above, showing limited power of contraction, vl, velum. 

 Fig. 26. Embryo with developing shell, stomach, and body cavity, st, stomach ; 

 be, body-cavity. 



Fig. 27. Further advanced embryo, -072 mm. in diameter, with stomodaeum 

 invaginating. sd, stomodseum. 



Fig. 28. Still further advanced embryo, with retractor muscles of the velum and 

 anal region, oesophagus, stomach, and intestine well defined, am, pm, velar muscles ; 

 ces, oesophagus ; i, intestine. 



Figs. 29 and 30. The velum of above respectively expanded and half-contracted. 



Fig. 31. Embryo '134 mm. in diameter, and 12 days old. Intestine looped ; the 

 liver diverticula differentiated ; the anterior adductor of the valves taking shape. 

 Iv, liver ; aa, anterior adductor. 



* Barrois, Les Glandes du pied et les pores aquiferes chez les Lamellibranches, Lille, 

 1885. 



t Holman Peck, Quart. Jour. Mic. Science, lxv. 1877. 

 % Zeitschrift fur Wiss. Zool., 1885. 



