ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 223 



sections the enteric cavity is seen, with two flat lateral dilatations 

 rej)rcsenting the liver, and surrounded by a thick mass of indifferent 

 mesoderm-cells, from which, on each side, a strand of cells extends 

 backwards almost to the posterior margin of the shell. These 

 strands, probably identical with those described by Flemming as the 

 lateral wings of the anterior pad, and regarded by Schierholz as 

 ganglionic rudiments, are shown by Schmidt to have no connection 

 with the nervous system, but to represent the rudiment of the future 

 organ of Bojanus. (c) He describes a pair of peculiar, large, flat 

 muscle-cells, stretching, one on each side, across the body-cavity, 

 connected at the one end with the middle of the shell by means of 

 numerous fine processes, and at the other with the large cells of the 

 so-called embryonic mantle. These two flat mesoderm-cells lie 

 parallel to the longitudinal axis of the embryo, exhibit a fine longi- 

 tudinal striping, and undoubtedly serve, in closing the shell, to draw 

 inwards the hooked processes, and thereby to secure the firmer 

 attachment of the larvfe. 



II. The development of the parasitic embryo. — (a) As Schierholz 

 has shown, the rudimentary foot increases greatly in size, assuming 

 a blunt conical form, and gradually presses the " Mittelschildtasche " 

 and the oral invagination ever further forwards ; while by this growth 

 of the foot, the external margins of the two lateral pits (the rudi- 

 mentary branchiae) are drawn out longitudinally and separate into 

 several knob-like elevations, (h) The often noted early disappearance 

 of the byssus organ is accompanied also by that of the sensory cells, 

 the embryonic shell-shutting muscle, the great part of the embryonic 

 mantle, and the above described muscle-cells. The adult adductors 

 are entirely new structures, (c) He confirms Braun's interesting 

 observation that the large conical cells of the mantle gradually 

 contract into a "mushroom-shaped" body, which lies in close 

 proximity to the fin-ray, and is concerned with the dissolving and 

 absorbing of the lime-salts required by the young mussel for the 

 growth of the shell, (d) The endodermic archenteric sac enlarges 

 along the middle line of the body, and comes into communication 

 with the now anterior oral invagination, while posteriorly, an anal 

 opening is formed by rupture and not by invagination. From the 

 " Mittelschildtasche," not only the mouth-opening, but the whole 

 fore-gut arises, and is therefore, unlike the other portions of the canal, 

 of ectodermic origin. The comparatively inconspicuous diverticula 

 of the archenteron, which represented the liver, increase greatly in 

 size, and grow out into two cylindrical sacs, lying parallel to, and 

 afterwards enveloping the alimentary canal, (e) The whole nervous 

 system is developed from the ectoderm ; the ganglia arise inde- 

 pendently, and at different epochs, as solid thickenings of the 

 epithelium ; the pedal are at first in connection with an invagination 

 which forms the hijssus- gland. Herr Schmidt first observed the 

 auditory sacs on the ninth or tenth day of parasitic life as invaginations 

 of the external epithelium on each side of the foot, afterwards sinking 

 inwards to the pedal ganglia as round masses, in which a distinct 

 lumen is, at a later stage, recognizable. (/) The origin of the gills 



