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Appendices to Fifth Annual Report 



gland, which arises as a distinctly refractive body near the velum (PL 

 XII. fig. 23, sh). The shell-gland soon assumes a bilateral form (PL 

 XII. fig. 24, sh), and its component cells may be made out (PL XII. 

 tig. 26, sh). After the valves have become differentiated, the stomodaeum 

 is seen to invaginate (PL XII. fig. 27, sd), but whether at the point 

 formerly occupied by the blastopore or not has not yet been settled. The 

 velum comes to be placed at the anterior region. It is relatively very large. 

 For a considerable period the shell-valves are too small to accommodate 

 the whole of the embryo (PL XII. fig. 27). When they have grown 

 sufficiently to do this, the velum, which at an early age exhibited ability 

 to fold together (PL XII. fig. 25, vl), can now be infolded and drawn 

 within the valves. The movement is accomplished by transparent muscular 

 bands, having their origin in the dorsal region of the valves. When the 

 velum is fully protruded, it projects laterally far beyond the external 

 margins of the valves, the latter being brought so closely together as to 

 leave only a narrow slit between them. The velum is excessively 

 sensitive, being withdrawn with a jerk the instant au obstacle is touched 

 by it. Simultaneously with the formation of the stomodceum, the 

 stomach, proctodeum, and body-cavity become defined. The stomach 

 is first distinguished as an oval body, composed of spherical closely-set 

 cells. The limiting tissue intervening between the velum and hinge, 

 anteriorly and posteriorly, is clothed with active, minute cilia. The 

 oesophagus (stomodaeum) becomes ciliated soon after its formation, and 

 the stomach and intestine follow suit. The body-cavity enlarges, and 

 the stomach having acquired muscular power, may be moved about 

 independently within it. The velum in embryos of two days and three 

 hours has two pairs of retractor muscles visible. They are triangular or 

 bifurcate at their extremities. The posterior pair are the stronger (PL XII. 

 fig. 28, pm). A pair of similar band-like retractors are inserted in the anal 

 region (PL XII. fig. 28, m). They are all highly contractile. In complete 

 contraction, they may form loops or present a beaded appearance. The 

 fully expanded velum consists of loose tissue, greenish by transmitted light. 

 The cirri are strong and regularly arranged. The long flagellum arises 

 from a central mass of tissue (PL XII. fig. 29, /). It is capable of inde- 

 pendent movement. A clearly-outlined arch-like excavation, concave out- 

 ward, is seen in the expanded velum. The dome of sectodermal tissue 

 external to it is, in other forms, associated with the supra-cesophageal 

 ganglion* (PL XII. fig. 31, d). When the velum is about to be retracted, 

 the cirri are bent inwards towards the centre (PL XII. fig. 30), and the 

 margins of the velum approximated so as to enclose them. The pallial 

 muscle is early developed as a band of considerable width running 

 round the whole free margin of the valves. The valves themselves are, 

 at an early period, finely sculptured or pitted. At the stage at present 

 under consideration they are almost semicircular, the hinge-line being 

 straight. There is a noticeable bulging of the curve of the free margin 

 anteriorly — that is, in the velar region (PL XII. figs. 28 and 31). The 

 valves are transparent, and already somewhat brittle. A few large scattered 

 mesoblast cells are found in the posterior dorsal region, and a few similar 

 cells in front of the velum. The latter (PL XII. fig. 31, aa) are the pre- 

 cursors of the anterior adductor muscle, which is the first to be differenti- 

 ated, not the posterior, as stated by Lacaze-Duthiersf and Balfour.;}; 

 Diverticula, one on each side of the stomach, are the rudiments of the 

 liver. They are composed of loosely-grouped, spherical cells, and early 



* Hoek (European Oyster), loc. cit. 

 t An. Sc. Nat., scr. iv. torn. v. p. 21. 

 X Loc. cit. p. 21 5. 



