Miscellaneous. 44] 
The walls of the stomach are differentiated into two layers—an 
external one of muscular fibres, and an internal mucous layer; this 
latter produces five horny teeth, preceded sometimes by the appear- 
ance of a single larval plate. The vitelline sacs, of which there are 
two at first, unite into one in the Orthoconcha. This sac, which 
opens into the dorsal part of the stomach, is absorbed and diminishes 
rapidly in the Hyaleacez ; on the contrary, it is developed in the 
Styliolaceze and the Creseidese, where it seems to play, provisionally, 
the part of the liver. In every case it diminishes in proportion as 
the liver is developed. The liver is composed of small diverticula of 
the wall of the stomach. The nutritive sacs have nothing to do with 
the formation of this organ. . 
The otocysts are formed early, in the midst ofa layer produced by 
a doubling of the ectoderm still composed of large embryonie cells. 
The otolith originates in the thickness of the wall of the vesicle, 
and falls afterwards into its cavity. In the Limaces and the Cepha- 
lopoda the otocyst is formed by an invagination of the ectoderm 
already composed of very small cylindrical cells. The size of the 
embryonic cells of the generative layer seems to be in this case, as 
in many cthers, the cause which determines the mode of formation 
of an organ by invagination or by simple folding. 
The nervous system is composed of a cephalic nervous mass and of 
a subcesophageal mass. The formeris formed by a double invagina- 
tion of the ectoderm of the cephalic region in the area circumscribed 
by the velum ; the mode of formation of the second has not been 
observed in the Pteropoda. 
The appearance of the shell is preceded by the formation of an 
invagination of the ectcderm a little in front of the aboral pole. 
This preconchylian invagination turns round; and the first rudiment 
of the shell appears on the projection thus formed. In exceptional 
or abnormal cases this invagination does not turn round, or rather it 
is reformed after having disappeared. Its existence is incompatible 
with that of an external shell and vice versd. It is the point of de- 
parture of the band which secretes the shell ring by ring, and which 
becomes the margin of the mantle. The first part of the shell, that 
which the larva inhabits, often differs from the portion which is 
added later on ; it may persist, fall or break off; and it has furnished 
me with characters which have enabled me to subdivide the sub- 
order of the Thecosomatous Pteropeda. The existence of the pre- 
conchylian invagination cannot be satisfactorily explained by purely 
physiological causes; it seems, then, to have hereditary causes, and 
may morphologically be compared to the conchylian invagination of 
the mollusks with internal shells, which invagination I have studied 
in Sepiola and the imax. The existence and signification of that 
invagination in the Cephalophora, the Cephalopoda, and the Lamel- 
libranchiata have been gradually cleared up by Lereboullet, Semper, 
Salensky, Ray Lankester, and myself. 
The sexual products originate at the expense of the endoderm. 
Sexuality can only be attributed to one embryonic lamella.— Comptes 
Rtendus, January 18, 1875, p. 196. 
Ann. & Mag. N. list. Ser. 4. Vol. xv. ol 
