PHYLOGENY OF THE PELECYPODA. 307 



but it may not be amiss to repeat the observations as my specimens, growing on transj^arent 

 glass, were very clear. In PI. xxiv, fig. 4, the dotted outlines of the alimentary canal are 

 those portions not visible from the right side, but seen when the oyster was viewed from 

 the left side through the glass microscope slide to which it was attached. From the 

 upper part of the hood-shaped pal[)s a short oesophagus, ce, leads directly into the stom- 

 acn, sf. The stomach, embedded in greenish-brown liver, has a circuitous outline espe- 

 cially on its ventral border. The intestine leads from the posterior end of the stomach 

 and extends in a nearly direct line to a point half way past the adductor muscle. It then 

 takes a shai-p bend and, retui-ning on itself, curves and follows the dorsal outline of the 

 body; again turning when near the umbos, it follows the outline of the body and passes 

 to the left of the oesophagus. It is then produced posteriorly in a curving line, passing to 

 the left at the two points where it crosses the earlier portion of its own track and emerges 

 i'rom the liver mass just above the heart. It passes outside of the heart and wrajijiiug 

 itself around the adductor muscle terminates in close relation to that muscle. This 

 course of the alimentary canal is identical with that figured by Rydei- (60), in an adult, 

 excepting that in the adult the oesophagus is directed more anteriorly than in the spat. 

 The faeces, of a peculiar, nearly horse-shoe shape,^ pass from the intestine in small par- 

 ticles and ai"e carried from the cloacal space by the excurrent water from the gills 

 through the opening, s, PI. xxiv, fig. 4, formed by a sinuosity of the border of the right 

 valve. All water from the body pa?ses ont through this opening, as indicated by arrows, 

 and the mantle is retracted to allow of its passage. 



A special excurrent passage is characteristic of youngei- and older spat than the one 

 described (PL xxv, figs. 5-6) ; but no trace of it has been found in the adult. The sinu- 

 osity of the right valve is apparently due to the constantly i-etracted condition of the 

 right mantle border at this point. It is constantly retracted to admit of the passage of 

 the excurrent water and being so retracted, as a necessary result, the shell is not built 

 at this area as rapidly as at other places where the mantle is fully extended. It might 

 be considered that the i)eeuliar excurrent opening was due to the mechanical conditions 

 under which the oyster lives, an opening being necessary on account of the close rela- 

 tion of the shell to the object of fixation, and it is possible that this is the case; but 

 against it may be urged the fiict that no such character is foiuid in Anomia which is sub- 

 ject to similar conditions. I think that this excurrent passage and the correlated sinu- 

 osity of the right valve are of hereditary significance. Examining the fossils, Ave find a sim- 

 ilar sinuosity of the left and i-ight valve charactei-istic in marked degree of the genus 

 Grypha^a; it is also found frequently in Exogyra and in man}^ fossil members of the Avic- 

 ulidaa, from which last grouj) the Ostreadre were doubtless evolved.^ 



As in the case ])resented, many peculiarities of the form of shells can doubtless be 

 elucidated by a study of the soft parts and life-habits of the animals. The byssal notch, 

 ridges of large siphonated forms as the Myidaj, and siphonate processes of Gasteropods 

 ai-e examples of such characters, many of which are specially studied in this jjaper. 



' The horse-shoe shape is the form of tlie cross sec- forms in the line of evolution of the gr(nip. I do not 



tion of the faeces and owes its origin to the peculiar form think this militates against what is above stated. Ostrea 



of the intestine Ryder ((JO). has an excurrent sinuosity of the valves similar to that 



'In section VII, it is stated that Grypha;a and Exogyra are found in Gryph;ea and Kxogyra, in all, inlierited (if atall) 



probably aberrant members of the OstreadiC, not typical from far-oft' ancestral groups. 



