298 PROCEKDIXGS OF THE MALACOLOGICAL SOCIETY. 



by Iluxlcy alone, and by the latter especially with a view as to the 

 jirescnce or absence of a posterior adductor muscle, I could not help 

 entertaining some doubt as to Mr. AVebb's figures. 



These specimens, being under my care, have often been examined by 

 me; but 1 made a further examination of them with Mr. Webb, and 

 have subsequently made a careful and renewed study of them, and 

 I here append a short account of them, since the conclusions to which 

 I have come differ in toto from those put forward by Mr. "Webb. 

 Horst's black spat are stained with hsenuitoxylin, and one specimen 

 shows more details than any other I have examined. Huxley's 

 specimens are stained with eosin, and, in spite of the diffuse nature 

 of the stain, show almost the entire structure. Fig. 1 is a camera 

 lucida drawing made from the best specimen on Horst's slide : this is 

 certainly not the one from Avhich Huxley's drawing was made. The 

 velum (r.) is well protruded, but its cilia are not so distinct as in 

 other specimens ; in the centre of this structure is seen a depression 

 and epidermal thickening, the supra-oesophageal ganglion (c.^.) ; below 

 the velum is the mouth («*.), leading into a broad oesophagus, which 

 in turn communicates with a large stomach {st.), partially subdivided into 

 two chambers, an anterior one receiving the oesophagus and the paired 

 hepatic diverticula {hd.), while the posterior gives origin to the 

 intestine (?'.), which runs back, then upwards and forwards, to the left of 

 the stomach, as in the adult {vide Horst, Huxley, and Jackson), not 

 to the right as figured by Webb, and then with a slightly sinuous 

 course the intestine runs down to the small pallial cavity {p.c), 

 where it opens by an anal orifice. Between the mouth and anus is 

 a lobe of the body capable of being protruded from between the valves : 

 this is the future molluscan foot (/.). In optical section a slight 

 pit-like depression and thickening of the epidermis may be seen : this 

 is obviously the developing, but transitory, pedal ganglion. The 

 edge of the mantle and its continuity with the dorsal outline of the 

 body is seen as a slightly stained layer of epiblast just within the shell. 



The retractor velar muscles appear as delicate bands of fibres 

 originating from the curvature of the shell near its dorsal margin 

 by two heads, and inserted among the cpiblastic cells of the velum. 

 There are four of these muscles, viz., a pair of anterior or dorsal, and 

 a pair of posterior or ventral ones. The anterior ones {a.v.r.) lie on 

 either side of the stomach, while the posterior ones {p.v.r.) start 

 close to the anus, and, skirting the edge of the pallial cavity, pass on 

 either side of the oesophagus to the velum. In connection with these 

 may be seen some minute pedal retractors. 



The functional adductor at tliis stage is situated dorsally to the 

 stomach, exhibiting in its relation to the shell all the characteristic 

 features of the anterior adductor of the typical di-myarian lamelli- 

 branch. In all the specimens which I have examined the muscle at 

 this stage is quite free and distinct from the epiblast of the anterior 

 body wall. When seen in surface view the fibres composing this 

 muscle exhibit polygonal ends abutting against the under surface of 

 the shell, but tlieir fibrous nature is best seen in specimens viewed 

 from the end or from below (Figs. 2 and 3). The muscular fibres 



