POLYZOA AND TUNICATA. 169 



ing portion, or withers, and is thrown off gradually in shreds. The 

 shaping of the internal organs out of the yolk-mass takes place very 

 rapidly, so that by the end of the second day of the sedentary state the 

 outlines of the branchial sac and of the stomach and intestine may be 

 traced; no external orifices, however, being as yet visible. The pulsation 

 of the heart is first seen on the third day, and the formation of the 

 branchial and anal orifices takes-place on the fourth; after which the 

 ciliary currents are immediately established through the branchial sac 

 and alimentary canal. The embryonic development of other Ascidians, 

 solitary as well as composite, takes-place on a plan essentially the same 

 as the foregoing, a free tadpole-like larva being always produced in the 

 first instance. ' 



560. This larval condition is represented in a very curious adult free- 

 swimming form, termed Appendicularia, which is frequently to be taken 

 with the Tow-net on our own coasts. The animal has an oval or flask- 

 like body, which in large specimens attains the length of one-fifth of an 

 inch, but which is often not more than one-fourth or one-fifth of that 

 size. It is furnished with a tail-like appendage three or four times its 

 own length, broad, flattened, and rounded at its extremity; and by the 

 powerful vibrations of this appendage it is propelled rapidly through the 

 water. The structure of the body differs greatly from that of the Asci- 

 dians, its plan being much simpler; in particular, the pharyngeal sac is 

 entirely destitute of ciliated branchial fissures opening into a surrounding 

 cavity; but two canals, one on either side of the entrance to the stomach, 

 are prolonged from it to the external surface; and by the action of the 

 long cilia with which these are furnished, in conjunction with the cilia 

 of the branchial sac, a current of water is maintained through its cavity. 

 From the observations of Prof. Huxley, however, it appears that the 

 direction of this current is by no means constant; since, although it 

 usually enters by the mouth and passes out by the ciliated canals, it 

 sometimes enters by the latter and passes out by the former. The caudal 

 appendage has a central axis, above and below which is a riband-like layer 

 of muscular fibres; a nervous cord, studded at intervals with minute 

 ganglia, may be traced along its whole length. By Mertens, one of the 

 early observers of this animal, it was said to be furnished with a peculiar 

 gelatinous envelope or H*aus (house), very easily detached from the body, 

 and capable of being re-formed after having been lost. Notwithstanding 

 the great numbers of specimens which have been studied by Miiller, 

 Huxley, Leuckart, and Gegenbaur, neither of these excellent observers 

 has met with this appendage; but it has been since seen by Prof. Allman, 

 who describes it as an egg-shaped gelatinous mass, in which the body is 

 imbedded, the tail alone being free; whilst from either side of the 

 central plane there radiates a kind of double fan, which seems to be 



1 The study of the development of Ascidians has derived a new interest and 

 importance from the discovery made by Kowalevsky in 1857, that their free- 

 swimming larvae present a most striking parallelism to Vertebrate embryoes, in 

 exhibiting the beginnings of a spinal marrow and a spinal column; thus bridging 

 over the gulf that was supposed to separate them from Invertebrata, and (when 

 taken in connection with the curious Ascidian affinities of Amphyoocus, the low- 

 est Vertebrate at present known) affording strong reason to believe in the deriva- 

 tion of the Vertebrate and Tunicate types from a common original. See his 

 Memoir Entwickelungsgeschichte der einfachen Ascidien,' in " Mem. St. Petersb. 

 Acad. Sci.," Tom. x., 1867, and the abstract of it in "Quart. Journ. Microsc. 

 Sci.," Vol. x., N.S. (1870), p. 59; also Prof, Haeckel's " History of Creation," Vol. 

 ii., pp. 152, 200. 



