xvii PEOTOCHORDATA 585 



We may therefore note that the Tornaria larva bears, in many 

 respects, a strong resemblance to the Bipinnaria larva of Asteroidea, 

 and, in external appearance, to the Auricularia larva of Holothuroidea; 

 from both of which it differs principally in possessing the posterior 

 ciliated band and the well-developed apical plate. The apical plate, 

 however, turns up in the Crinoid larva and in the late Echinnpluteus 

 larva, and it was probably once a feature of all Echinoderm 

 larvae. The tendency for the longitudinal ciliated band to rearrange 

 itself in transverse ciliated bands is exemplified by the late Echino- 

 pluteus larva, by the "Pupa" of Holothuroidea, and by the Crinoid 

 larva. The posterior ciliated baud of the Tornaria larva may be 

 the result of such a rearrangement. We arrive, finally, at the con- 

 clusion that no important difference divides the Tornaria from the 

 Echiuoderm type of larva ; the great difference between the two lies 

 in the nature of their metamorphoses. 



It follows that Echinodermata and Enteropneusta (and through 

 the latter the whole of the Vertebrata) are descended from the 

 same stock of simple free-swimming animals. In this ancestral 

 stock the coelom was already divided into three sections on each 

 side, such as become delimited in the coeloni of the Echinoderm 

 larva as growth proceeds; and the middle sections, called 

 hydrocoeles in Echinodermata, and collar-cavities in Enteropneusta, 

 were produced into ciliated tentacles. 



The main stem of the common stock which, in its day, must 

 have constituted a dominant type of animal, kept to the sea and 

 gave rise to the higher Vertebrata ; one division of which, the Pisces, 

 still dominate that element. An offshoot, which was destined to 

 give rise to both Echinodermata and Enteropneusta, dropped to the 

 bottom and took to gliding over the soft ooze. Certain of these 

 gliders finally fixed themselves to the substratum by the prae-oral 

 lobe, and gave rise to the Echinodermata ; whilst others degenerated 

 into burrowing habits and became Balanoglossida. But still others 

 seem to have learned to fix themselves by the ventral integument, 

 and thus gave rise to the other division of Enteropneusta known as 

 the Cephalodiscida. In these last, as. in the Echinodermata, the 

 middle or collar body -cavities are prolonged into ciliated tentacles ; 

 in fact, the great difference between them and the Echinodermata 

 (apart from the part of the body with which they fix themselves) 

 is that in the Cephalodiscida both collar-cavities are equally developed, 

 whereas in Echinodermata the left overpowers the right, and leads 

 to that peculiar ring-shaped growth of the left hydrocoele which 

 later imposes a radial symmetry on the primitive bilateral symmetry. 

 It is quite conceivable that Brachiopoda also, as indicated by the 

 three segments of their larva, may be distantly related to the same 

 type, but the substantiation of this suggestion would require a 

 great deal of further work. 



If the reasoning outlined above be sound, a most interesting 

 conclusion can be drawn as to the origin of the peculiar Vertebrate 



