TUNICATA I APPENDICULAR!*.. 591 



the branchial sac and alimentary canal. — The Embryonic Develop- 

 ment 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.* 



457. 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. This 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 Ascidians, its 

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

 entirely destitute of ciliated branchial fissures opening into a sur- 

 rounding 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, maybe traced along its whole length. — By Mertens, 

 one of the early observers of this animal, it was said to be fur- 

 nished with a peculiar gelatinous envelope or Haus (house), very 

 easily detached from the body, and capable of being re-formed after 

 having been lost. Notwithstanding the great numbers of speci- 

 mens which have been studied by Muller, Huxley, Leuckart, and 

 Gregenbaur, neither of these excellent observers has met with this 

 appendage ; but it has been recently 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 formed by a semicircular membranous lamina folded upon 

 itself. It is surmised by Prof. Allman, with much probability, 

 that this curious appendage is ' nidamental,' having reference to 

 the development and protection of the young ; but on this point 



* For more special information respecting the Compound Ascidians, 

 see especially the admirable Monograph of Prof. Milne Edwards on that 

 group ; Mr. Lister's Memoir ' On the Structure and Functions of Tubular 

 and Cellular Polypi, and of Ascidise,' in the "Philos. Transact.," 

 1834 ; and the Art. Tunicata in the " Cyclopedia of Anatomy and Phy- 

 siology." 



