ANATOMY AND PHYSIOLOGY. 
Moll. 41 
two cells which were previously separated from the large yolk-cell, 
although the possibility is not denied that they may be of ectodermic 
origin. The blastopore is in the form of a slit, which becomes com¬ 
pletely closed, but whether from before or behind was not seen. The 
primitive enteron ends anteriorly in a mass of cells, posteriorly it remains 
in contact with the ectoderm, and the oesophagus arises by invagination ; 
a comparison is instituted between the gastrulation of Lamellibranchs and 
of Gastropods. „ The Trochophore is described at length : the oesophagus 
passes upwards and backwards, and leads into a dilated stomach, which 
has two lateral anterior processes, the future liver ; on either side the 
mouth is a triangular ciliated area, which is manducatory in function, 
and homologous with a similar tract in Polygordius. Resembles Gastro¬ 
pods in the early formation of the foot; the cavity of the Trochophore is 
a “ primitive schizocoele.” The kidney arises as a ciliated tube, which 
runs from before downwards and backwards, and leads into a large cell, 
which is probably perforated by it ; anteriorly the canal is expanded into 
a funnel, but its continuity with the primitive schizocoele was not demon¬ 
strated. The byssus gland is formed by two invaginations in the hinder 
portion of the foot, and retains traces of its primitive bilateral symmetry 
even when closed. The liver-sacs exhibit periodic contractions at inter¬ 
vals of about fifteen seconds. The pericardium is developed from a small 
vesicle formed in the mesoderm above the genital cells on either side ; 
behind each of'these a number of mesoderm cells form a canal—the 
organ of Bojanus—the lower of which opens into the vesicle just men¬ 
tioned ; afterwards this canal is so directly continuous with the ectoderm 
that it appears to have arisen from it, which, indeed, is not impossible; 
it consists of three portions—(1) a short ciliated, (2) a long glandular, 
and (3) an efferent. The two pericardial vesicles unite both above and 
below the intestine, and their inner (median) walls form the wall of the 
ventricle ; their cavity is regarded as a “ secondary schizocoele,” and the 
pericardium is not a part of the vascular system. The ganglia all arise 
from the ectoderm, and the visceral are the latest to develop. The 
structure of the adult gill is described. 
Schmidt (325, 326) has infected fish with the larvae of Anodonta, and 
has thus been able to throw light on their development during the para¬ 
sitic stage. He agrees with Forel and Schierholz that the “ Mittelschild- 
tasche ” of Flemming marks the posterior end of the embryo. ‘On either 
side of the intestine, which is surrounded by a mass of indifferent tissue, 
is a hepatic dilatation. Two bands of cells, one on either side, are the 
rudimentary organs of Bojanus, not nervous structures. A peculiar 
muscle-cell, attached to the shell by numerous fine processes, lies parallel 
to the long axis of the embryo, and serves to attach it to the host. 
Immediately after the attachment of the Glochidium, the future foot 
becomes elevated as a blunt cone, and forces the oral invagination for¬ 
ward, whilst the margins of the lateral grooves, the future gills, become 
elongated, and ultimately separate into several papillae. Many embryonic 
structures (byssus gland, sensory cells, adductor muscle, and mantle) now 
disappear. Certain cells of the mantle form the “ mushroom-shaped 
body ” of Braun, which is charged with the supply of calcareous matter 
