TUNIC AT A 9 1 7 



much the appearance of a tadpole, the tail being straightened 

 out, and propelling the body freely through the water by its lateral 

 strokes. The centre of the body is occupied by a mass of liquid yolk, 

 and this is continued into the interior of three prolongations which 

 extend themselves from the opposite extremity, each terminating in a 

 sort of sucker. After swimming about for some hours with an active 

 wriggling movement, the larva attaches itself to some solid body by 

 means of one of these suckers ; if disturbed from its position, it at 

 first swims about as before ; but it soon completely loses its activity, 

 and becomes permanently attached ; and important changes manifest 

 themselves in its interior. The organs and tissues which constitute 

 the chief part of the future animal are gradually drawn back, so that 

 the whole of it is concentrated ftito one mass ; and the tail, now con- 

 sisting only of the gelatinous envelope, is either detached entire from 

 the body by the contraction of the connecting portion, or withers, 

 ami is tin-own 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 

 ciu-rents 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 with the curious exception of some species of 

 Molgula. 1 



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

 swimniiiig form, termed Appeiidicularia, 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 dif- 

 fers 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 surrounding cavity ; but two canals, 



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

 portance from the discovery, made by Kowalevsky in 1867, that their free-swimming 

 larvae present a most striking parallelism to vertebrate embryos, in exhibiting the 

 beginnings of a spinal marrow and a notochord ; thus bridging over the gulf that was 

 supposed to separate them from Invertebrata, and (when taken in connection with 

 the curious Ascidian affinities of Amphioxus, the lowest vertebrate at present known) 

 affording strong reason for belief in the derivation of the vertebrate and tunicate 

 types from a common original. See his memoir ' Entwickelungsgeschichte der 

 einfachen Ascidien ' in Mem. St. Petersb. Acad. Sci. torn. x. 1867, and the abstract 

 of it in Quart. Journ. Microsc. Set. x. n.s. 1870, p. 59 ; also Professor Haeckel's History 

 of Creation, ii. pp. 152, 200. Further information will be found in chap. ii. of vol. 

 ii. of the late Professor Balfour's Comparative Embryology, and an application of the 

 facts of development to the philosophy of the subject in Professor Ray Lankester's 

 Degeneration (London, 1880). 



