220 THE ACEPHALA. $ 200. 
ual elongation of the posterior part of the body of the embryo, but is pro- 
duced by the fusion of a series of globules which result from the vitelline 
segmentation. These globules lie upon the surface of the embryo and, in 
their separation from it, assume a tail-like body which is folded in front, 
and only latterly is extended out behind. With some of the compound 
forms, it forms also two eye-specks upon the back of the larva. At this 
eriod of development the eggs are still in the cloaca, or perhaps have been 
discharged through the anal tube. Subsequently, the embryos rupture 
their shell, and then swim freely about by means of their very active 
tails. Soon after this, the larvae are completely surrounded by a transpa- 
rent structureless envelope, which ultimately becomes the mantle. They 
are then fixed by their anterior extremity, — lose their tail and assume 
their adult form. 
With the compound forms, before the larvae have become fixed and 
deprived of their tails, numerous button-like prolongations arise from the 
anterior extremity and extend into the mantle; these, after the fixation of 
the embryo, are changed into as many individuals. 
The development of the Salpaehas yet been incompletely observed, for 
its earlier conditions have received no attention. But the later ones pre- 
sent very curious phenomena. 
In the first place, it is quite remarkable that the two forms of these ani- 
mals which are always viviparous, produce young wholly dissimilar. The 
solitary individuals produce others joined together in a chain-like manner, 
while these last give rise again to the solitary forms. But in neither case 
do the embryos undergo a metamorphosis. This chain of individuals is 
usually composed of two rows joined together by several cords and envel- 
oped in a common membranous tube. The individuals at the anterior 
extremity of this tube are the more developed, — there being a gradation 
in this respect to the posterior extremity, where they appear only as simple 
punctiform bodies. This tube usually surrounds also the nucleus of the 
parent, into the cavity of whose body its anterior extremity often widely 
observed by Savigny (Mém. &c. Pl. XI. fig. 23, 
PL. XXI. fig. 1.) with Clavelina and Botryllus. 
Subsequently they have been described by Au- 
douin and Milne Edwards (Ann. d. Sc. Nat. 
XV. 1828, p. 11); Sars, Beskrivelser, &c., p. 69, 
Pi. XIL.), and Dalyell (Edinb. new Philos. Jour. 
Jan. 1839, p. 153). Latterly Milne Edwards (Sur 
les Ascidies, &c., loc. cit.) has furnished an exact 
«mbryology of these Ascidians, but which has been 
completed by Van Beneden (Mém. sur ’embryog. 
Panat. et la physiol. des Ascid. loc. cit.), ‘and by 
Kolliker (Ueber das Yorkommen der Holzfaser im 
Thierreich loc. cit.). 
(Additional note) The memoirs cited above 
lave since been published, that of Van Beneden 
in Mem. delAcad. de Bruxell. XX. 1847, Pl. IL. 
TIL.; that of Aeldiker in Ann. d. Sc. Nat. V. 1846, 
217, Pl. VII. 
3"Amaroucium and Aplidium, according to 
Kélliker, and as confirmed by Van Beneden. 
* [§ 200, note 5.] The embryology of the Ascidiae 
has been followed out by Agassiz (Proceed. Amer. 
Soc. for the Advancem. of Sc. 2d meeting, 1849, 
Cambridge, p. 157), and by Krohn (Muiller’s Arch. 
1852, p. 312). The observations of Agassiz are 
complete throughout, but unfortunately not yet all 
4 For the development of a simple Ascidian, see 
Dalyell, loc. cit. 
5 Atcording to Milne Edwards (loc. cit. p. 30), 
these animals use these processes like suckers to fix 
themselves. But this is contradicted by the obser- 
vations of A’élliker, and Van Beneden.* 
6 This mode of propagation first described by Cha- 
misso (loc. cit.), has been doubted by Eschricht, 
who thinks that the young Sa/pae produce solitary 
foetuses, while those of a more advanced age pro- 
duce the aggregated form. But, as Steenstrup (Ue- 
ber den Generationswechsel, p. 36), has justly ob-. 
served, there is no observation to support this view. 
The alternate generation of theSalpae, as first 
described by Chamisso, has been confirmed in all 
particulars by Sars, and Krohn (loc. cit.). Itis how= 
ever, singular that, according to Krohn, the single 
egg of the aggregate Salpae is formed in an ovary, 
while the whole development of these same animals 
when solitary occurs from an internal gemmation. 
published ; the published portion (loc. cit.) refers 
more particularly to the formation and intimate 
structure of the egg. Those of Krohn are chiefly 
confirmatory of those of Edwards and other ob- 
Servers above mentioned. — Ep. 
