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 pinious, which characterize the embryos of the Branchiata, is also 

 incomplete. 



The development of tbe 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 Sagitta, as far as yet known, differs essentially from 

 that of the Gasteropoda/®' in that its embryo is not formed at the expense 



Vm.' 1837, p. 129, PI. Til. IV ; Pouch et, Ann. d. vitellus of the Limacina, for Dujardin (Ann. d. Sc. 



Sc. Nat. X. 1838, p. 63 (Lymnaeiis) ; and Rathk^, Nat. VII. 1837, p. 374, or, Observ. au Microsc. Atlas, 



Frortep's neue Not. XXIV. 18i'2, p. 161 {Lym- 1842, PI. V. fig. 10, 11) has seen in the eggs of Li- 



naeus, Planorbis, and Helix). max cinereus soon after their deposition, singular 



6 See Laurent, in the Ann. d. So. Nat. IV. 1835, vitelline movements exactly resembling the alter- 

 p. 248 {Limax and Ar ion) ; Van Bcnerfen, and nate protrusions and retractions of the parenchyma 

 Windischmann, in the Bull, de I'Acad. roy. de of Amoeba. 



Bruxelles, V. No. 5, p. 286, Ann. d. tSc. Nat. IX. 8 See the observations of Darwin, in the Ann. 



1838, p. 366, and in Mü/ler's Arch. 1841, p. 176, of Nat. Hist. XIII. p. 4, or Ann. d. Sc. Nat. I. 



Taf. VII. Vlir. (Umax).* 1844, p. 363.t 



7 This contractihty shows itself quite early in the 



* [ § 229, note 6.] See also O. Schmidt (Ueber most remarkable episode in the embryology of the 



die Entwickelung von Limax agrestis, in Muller^s Mollusca, the development of certain MoUusks in 



Arch. 1851, p. 278) who differs in many points from Holothurio'idea. The facts of the case were discov- 



Van Beneden and Windischmann, as to the his- ered and announced by J. Midler (Verhandl. der 



tological development of some of the organs. See, Akad. zu Berlin, 1851, p. 628 (October 23), and 



furthermore, Gegenbaur, Siebold and Kölliker''s Nachtrag, p. 679 (Nov. 13), or in extenso in Mill- 



Zeitsch. III. 1852, p. 371. — Eo. /er's Arch, 1852, p. 1), and they are mdeed so 



t [ § 229, note 8.] See, for some of the more re- wonderful that it is well they were first brought out 



cent contributions to the embryology of the Cephal- by so reliable a physiologist and embryologist. 



ophora, Koren and Danielssen (Bidrag til Pec- The main facts, briefly stated, are as follows : In 



tinibranchiernes Udvicklings histoire, Bergen, 1851, certain individuals of Synapta digitata there are 



or its Translation mto French in the Ann. d. Sc. found from one to three sac-like bodies in tlie cavity 



Nat. XVIII. 1852, p. 257, and XIX. 1853, p. 89), of the body, and attached by their superior extremity 



and Gegenbaur (Beiträge zur Entwickelungsge- to the head, and by the lower end to the intestine ; 



schichte der Landgastropoden, in Siebold and Köl- but this connection of the sac with the abdominal 



liker''s Zeitsch. HI. 1852, p. 371.) These works are and other organs, is one of simple contiguity and 



quite complete as far as they go, and that of Ge- not of very direct communication. The upper por- 



genbaur, especially, has full details upon the form- tion of the sac is of a yellow, and the lower of a 



ation of all the organs and their mutual embryolog- green color ; the lower portion, moreover, i« intus- 



ical relations. No just resum6 can be given in the suscepted, with a blind end, like an inverted finger 



proscribed limits of my notes. of a glove. It is in this sac-like organ that are de- 



I cannot here well omit at least an allusion to that veloped true MoUusks ; in the upper or more ca- 



