266 THE CEPHALOPHORA. $ 229. 
upon itself; its posterior extremity soon assumes a spiral form and is 
covered with an alveolate wrapper, upon which gradually appear the 
convolutions of the shell, without there being formed, at the same time, 
an operculum. During this period, the eyes, tentacles, border of the 
mantle, and the foot, appear at the anterior extremity; and, in the 
interior, the auditive capsules, the intestinal canal, the liver and heart 
are gradually developed. Here, therefore, the development of the 
cephalic pinions, which characterize the embryos of the Branchiata, is also 
incomplete. 
The development of the naked Pulmonata is quite different. When 
the round embryos begin to rotate, two crests appear side by side, upon 
the previously divided vitellus ; one of these is changed into the shield and 
into the respiratory and circulatory organs situated beneath, while the other 
goes to form the foot. At its anterior extremity, appear the eyes, tenta- 
cles, and lips; and at the posterior extremity, a peculiar contractile vesicle 
is formed. This vesicle presses its contents towards the vitelline substance 
which is still contained in a kind of vitelline sac projecting anteriorly 
between the two crests, and which, also, becomes contractile.” By this 
arrangement there is an interchange of the contents of the vitelline sac and 
the caudal vesicle, due to their alternate contractions. Subsequently, the 
liver and digestive canal are formed out of the vitelline substance between 
the two crests. 
The vitelline sac and caudal vesicle are in this way con- 
siderably diminished, and, at last, wholly disappear. 
The development of Sagztta, as far as yet known, differs essentially from 
that of the Gasteropoda,® in that its embryo is not formed at the expense 
VIII. 1837, p. 129, Pl. III. IV; Powchet, Ann. d. 
Sc. Nat. X. 1838, p. 63 (Lymnaeus) ; and Rathké, 
Froriep’s neue Not. XXIV. 1842, p. 161 (Lym- 
naeus, Planorbis, and Helix). 
6 See Laurent, in the Ann. d. Sc. Nat. IV. 1835, 
p. 248 (Limax and Arion); Van Beneden, and 
Windischmann, in the Bull. de Acad. roy. de 
Bruxelles, V. No. 5, p. 286, Ann d. Sc. Nat. IX. 
1838, p. 366, and in Muller’s Arch. 1841, p. 176, ' 
Taf. VIL. VIII. (Limaz).* 
7 This contractility shows itself quite early in the 
_ *[§ 229, note 6.] See also O. Schmidt (Ueber 
die Entwickelung von Limax agrestis, in Muiler’s 
Arch, 1851, p. 278) who differs in many points from 
Van Beneden and Windischmann, as to the his- 
tological development of some of the organs. See, 
furthermore, Gegenbaur, Siebold and Kélliker’s 
Zeitsch. III. 1852, p. 371. — Ep. 
t [§ 229, note 8.] See, for some of the more re~ 
cent contributions to the embryology of the Cephal- 
ophora, Koren and Danielssen (Bidrag til Pec- 
tinibranchiernes Udvicklings histoire, Bergen, 1851, 
or its Translation into French in the Ann. d. 8c. 
Nat. XVIII. 1852, p. 257, and XIX. 1853, p. 89), 
and Gegenbaur (Beitrage zur Entwickelungsge- 
schichte der Landgastropoden, in Siebold and Kél- 
liker’s Zeitsch. IIL. 1852, p. 371.) These works are 
quite complete as far as they go, and that of Ge- 
genbaur, especially, has full details upon the form- 
ation of all the organs and their mutual embryolog- 
ical relations. No just résumé can be given in the 
proscribed limits of my notes. 
I cannot here well omit at least an allusion to that 
vitellus of the Limacina, for Dujardin (Ann. d. Se. 
Nat. VII.1837, p. 374, or, Observ. au Microsc. Atlas, 
1842, Pl. V. fig. 10, 11) has seen in the eggs of Li- 
max cinereus soon after their deposition, singular 
Vitelline movements exactly resembling the alter- 
nate protrusions and retractions of the parenchyma 
of Amoeba. 
8 See the observations of Darwin, in the Ann. 
of Nat. Hist. XIII. p. 4, or Ann. d. Sc. Nat. I. 
1844, p. 363.+ 
most remarkable episode in the embryology of the 
Mollusca, the development of certain Mollusks in 
Holothurioidea. The facts of the case were discov- 
ered and announced by J. Muller (Verhandl. der 
Akad. zu Berlin, 1851, p. 628 (October 23), and 
Nachtrag, p. 679 (Nov. 13), or in extenso in Mul- 
ler’s Arch, 1852, p. 1), and they are indeed so 
wonderful, that it is well they were first brought out 
by so reliable a physiologist and embryologist. 
The main facts, briefly stated, are as follows: In 
certain individuals of Synapta digitata there are 
found from one to three sac-like bodies in the cavity 
of the body, and attached by their superior extremity 
to the head, and by the lower end to the intestine 5 
but this connection of the sac with the abdominal 
and other organs, is one of simple contiguity and 
not of very direct communication, The upper por- 
tion of the sac is of a yellow, and the lower of a 
green color ; the lower portion, moreover, is intus- 
suscepted, with a blind end, like an inverted finger 
ofa glove. It is in this sac-like organ that are de- 
veloped true Mollusks ; in the upper or more ca- 
