PMYLOGENY OF THE PELECYPODA. 357 



cleft lip to its extreme distal portion and the very young, fig. 5, in crawling extends and 

 flattens it against the object of support in a prehensile disc-like fashion, as described in 

 Pecten irradlans. Aftei- the animal becomes permanently fixed, figs. 1 and 2, the foot 

 was not observed to extend beyond the margins of the valves; but it was constantly moved 

 within the mantle walls in a sinuous manner. The foot is of lar^e volume embracin<r 

 the byssus at its point of attachment to the glass slide, fig. 2, and the open cleft of the 

 foot makes a V-shaped space immediately in front of the byssus; but soon the walls of 

 the cleft are in contact. The byssus is thus seen to originate from a cleft in the base of 

 the foot as in ordinary byssated Pelecypods. 



The upper pi'oximal face of the calcareous plug of adult Anomia is marked by lines 

 parallel to the longer axis of the shell and similar lines are noticeable in the basal portion 

 of the plug, hs, where attached to the glass, PI. xxix, fig. 2. The lines are made up of 

 j)arallel series of amalgamated, calcareous, discoid points seen in the fore part of the 

 byssal attachment in the figui'e. The discoid points ai'e dii-eetly comparable to the discoid 

 points of fixation of sepajate byssal threads observable in Pecten, Mytilus, etc., the differ- 

 ence being that in Anomia the several points are calcified. In the young we have there- 

 fore the fullest substantiation of Lacaze-Duthiers' homology of the plug of Anomia with 

 the byssus of other Pelecypoda; for we can actually see the byssus coming from the cleft 

 in the foot and the separate byssal threads amalgamating to form the calcareous plug. 

 The palps of Anomia, PI. xxix, figs. 1-2, originating close to the mnbos pass to the 

 right of the anterior i-etractor muscle and to the left of the foot and terminate in close re- 

 lation to the anterior limits of the gills. They are marked by plicated ridges on their pos- 

 terior borders. The coils of the intestine were not followed, being hidden in the dark 

 brown liver follicles which cover the greater part of the body mass. The liver presents 

 two regions of densest portion separated by a lighter portion in which there are few folli- 

 cles as shown in the figures. The i-ectum passes over the dorsal border of the adductor 

 muscle and is produced to a considerable extent into the cloacal chamlier, the anus,«, being 

 free, figs. 1-2, and in the adult a similar condition exists. A highly developed caecum, 

 c, fig. 2, as in the adult, is prolonged in a curved line from the body mass past the ven- 

 tral border of the adductor and to the right of the gills ending blindly in the cloacal cham- 

 ber. It is clear in the young as it is not complicated l)y the existence of the reproduc- 

 tive gland found in the adult. 



The heart, h, consists of a ventricle and probably two auricles as in the adult,^ but the 

 heart in the young is surrounded by a pericardium, whereas in the adult Lacaze-Duthiers 

 says that this is wanting. He exi)resses the belief that the pericardial walls may have be- 

 come united with the walls of the heart 25ro])er and in view of its early existence this is a 

 reasonable explanation of its disajjpeai-ance. On the left side a large vessel, v, fig. 1, passes 

 from the heart in front of the byssal muscle and to the gills of the left side; another vessel, 

 V, passes between the adductor muscle and retractor of the foot. These are similar to 

 those of the adult after Lacaze-Duthiers. Othervessels. ramify in the mantle as described 

 in yovuig oysters. The pulsations of the heart are variable. In fig. 1 were counted eighty 



' I express this -wiUi a degree of caution, as I did not parts beinj; so coincident -with Ids flgurcs and description, 



actually observe in the young the second auricle described its presence is assumed ; as he says it is with difficulty seen 



In the left side of the adult by Lacaze-Duthiers ; but other in the adult. 



HEMOIUS UOSTON SOC. NAT. HIST., VOL. IV. IS 



