220 TUE ACEPUALA. § 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. AVith some of the compound 

 forms, it forms also two eye-specks upon the back of the. larva.'"* At this 

 period of development the eggs are still in the cloaca, or perhaps have been 

 discharged through the anal tube. Subsecpiently, 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 

 exti-emity 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 (Mem. &c. PI. XI. ti^. 2^, 4 For the development of a simple Ascidian, see 



PI. XXI. fig. li.)witli C/avelina aud Botryllus. Dali/c/l, lie. cit. 



Subsequently tliey have been described by Aa- 5 According to Milne Edwards (loc. cit. p. 30), 



douin aud Milne Edwards (.\nu. d. Sc. Nat. these animals use these processes like suckers to fix 



XV. 1828, p. 11); .SVir.'!, Beskriveiser, &c., p. 89, themselves. But this is contradicted by the obser- 



Pl. XII.), and UaUjell (Edinb. new Phllos. Jour, vations of ft'6//(A-er, and *^'an Be/ieden.* 



Jan. 18ait, \i. 153). Latterly Milne Edwards (Sur U This mode of propagation firstdescribed by Cha- 



les Ascidies, &c., loc. cit.) has furnished an exact viisso (loc. cit), has been doubted by £sc/tr!cAY, 



embryology of these Ascidians, but which has been who tliinks that the young Si/paeiiroduce soliuiry 



completed by Van Beneden (.Mtm. sur I'embryog. foetuses, while those of a more advanced age pru- 



I'anat. et la physiol. des Ascid. loc. cit.), and by duce the aggregated form. But, as Steenstrup (Ue- 



Kiillilier (Ueber das Vorkommender Holzfaserim ber den Generationsweehsel, p. 30), has justly ob- 



'I'hierreich Vtc. cit.). served, there is n:> observation to support this view. 



[.\dditional note.] The memoirs cited above The alternate generation of thein/^/ae, as lirst 



iiuve since been published, that of Van Beneden described by ('/(am ia'.«o, has been confirmed in all 



in Mem. de I'Acad. dj Bru.xell. XX. 1847, PI. II. particulars l)y6V(r.«, and A'roAu (loc. cit.). It is liow- 



III.; that of KuUiker m Ann. d. Sc. Nat. V. 1846, ever, singular tliat, according to Krolin, the single 



217, PI. VII. egg of the aggregate 6a/pae is formed in an ovary, 



'iAmarouciiim and Aplidium, according to while the whole development of these same animals 



h'otli/cer, aud as confirmed by Van Beneden. when solitary occurs from an internal gemmation. 



*[§ 200, note 5.] The embryology of the .\scidiae published; the published portion (loc. cit.) refers 



has been followed out by Agassiz (Proceed. Amer. more particularly to the formation and intimate 



Soc. for the Advancem. of Sc. 2.i meeting, 1849, structure of the egg. Those of Krolin are chiefly 



Cambridge, ]). 107): and by A'roAn (.l/uWcr'i Arch, confirmatory of those of Edwards and other oU 



1852, p. 312). The observations of Ai^assiz are servers above mentioned. — Ed. 

 complete throughout, but uufortiuiately not yet all 



