440 Miscellaneous. 
aboral axis of the larva. The ectoderm closes up in the last place 
at the point of union of the three spheres, a point which may coin- 
vide either with the aboral or with the oral pole of the larva. I 
am in favour of the latter alternative. 
The embryonic development of the Gymnosomes forms a transition 
between that of the Thecosomes, which I have just recapitulated, 
and that of the Heteropoda, between the formation of the embryonic 
lamellee by envelopment and their formation by invagination. 
The digestive cavity is formed by a simple differentiation of the 
mass of nutritive or central cells. From this results a completely 
closed trilobate cavity. From the median lobe proceeds the diges- 
tive tube ; from the lateral lobes the nutritive sacs. The cells com- 
posing the walls of this cavity descend directly from the nutritive or 
central cells of the embryo; they are small and numerous round the 
median cavity ; cuneiform, and composed in great part of nutritive 
substance round the lateral cavities. The median portion lengthens 
to form the stomach and the intestine. An invagination of the ecto- 
derm, starting from the point where this lamella has closed up, de- 
scends to meet the stomach, with which it unites. This invagination 
represents the mouth and cesophagus; the point of junction the 
cardia. It represents in front a diverticulum which gives origin to 
the radula. This development of the digestive tube agrees point by 
point with what we know of the development of the Rotifera. 
The first cilia which appear are motory ; they are in small tufts 
on a circular zone on a level with the mouth; then a band of small 
cilia grows below the larger ones and serves to convey the nutritive 
particles to the mouth. 
The foot has its origin in a thickening of the ectoderm, which 
occupies the greater part of the ventral surface of the embryo. It 
afterwards takes the form of a hump, and then that of a horizontal 
tongue, which sometimes bears an operculum on its lower side. It 
divides into a median lobe and two lateral ones, which become the 
swimming-organs. 
The pallial cavity is formed by sinking-in of the ectoderm between 
the edge ot the shell and the neck of the larva, always on the right 
of the anus whatever may be the position of the latter. 
The larvee of the Pteropoda have two contractile sinuses, situated 
the one at the foot and the other in the dorsal region, which send 
from one to the other the liquid contained in the cavity of the body. 
Neither of these sinuses can be compared to those of the embryo of 
the Limaces. The cephalic sinus of the Zimaces corresponds to all 
the median portion of the velum and to the whole dorsal region of 
the embryos of the Pteropoda. The contractile sinus of the foot of 
the Limaces is situated at the extremity, and not at the base of the 
foot as in the Pteropoda. 
The kidney is formed at the expense of the ectoderm, and the 
heart by the differentiation of a mass of cells of the mesoderm. 
The internal aperture of the renal canal opens outside the heart and 
into the pericardium when the latter is afterwards formed. The 
kidney beats with almost as much vivacity as the heart. The aorta 
and the arteries are formed by the differentiation of chains of meso- 
dermic cells. 
