PRIMITIVE VERTEBRATES 



421 



The second type of larva, characteristic of certain species, is 

 known as a Tornaria (fig. 946). It resembles a trochosphere in 

 some respects, while in others it approximates to the larvae of star- 

 fishes, and was originally believed to be a young Echinoderm. 

 The uniform covering of short cilia and a posterior ciliary circlet, 

 often found in the first-named type, are also present here, but there 

 is in addition a complicated band of similar nature which passes 

 round the mouth, and takes the course shown in the figure. It is 

 comparable to the band of larval Echinoderms (pp. 355 and 357). 



The large group of SEA- SQUIRTS, ASCIDIANS, or TUNICATES 

 (UROCHORDA) presents a great variety of life-histories which are 

 of special interest, because they 

 illustrate quite a number of 

 general principles. We may 

 take as a first example a simple 

 fixed Tunicate (Ascidia), of 

 which the structure has already 

 been briefly described (see vol. 

 i, p. 297). The egg hatches out 

 into a tadpole-shaped larva (fig. 



947), Which pOSSeSSeS the three Fig. 947 .-Adult(A.) and Tadpole Larva (B.) of Simple 



1 "U ' "L J" * "U Ascidian (A. reduced and B. enlarged), g, Gill-slit; m, 



Characters WhlCh dlStingUlSh mouth; n, notochord; p, adhesive papillae. 



vertebrate animals from others. 



For it has (i) gill-slits, (2) a tubular central nervous system placed 

 near the upper surface of the body, (3) an elastic rod, the noto- 

 chord (the " forerunner of a backbone "), lying immediately below 

 the nervous system, though in this particular instance limited to 

 the tail-region. After leading a free-swimming life for some time 

 the larva fixes itself by three sticky knobs on its head, and 

 becomes an adult by a series of radical changes in its organization. 

 The tail, with its notochord, shrivels up the nervous system 

 becomes reduced to a small solid mass or ganglion and the gill- 

 slits become so numerous as to convert the pharynx into a sort 

 of ciliated basket, by which currents are set up which enable the 

 animal to feed and breathe. A firm protective covering (test) is 

 formed on the surface of the body, and the shape comes to re- 

 semble that of a skin flask with two openings, into one of which 

 water streams, afterwards flowing out again through the other. 

 The adult is therefore of lower grade than the larva, and only 

 exemplifies one of the three crucial vertebrate characters, i.e. the 



