xvii PROTOCHORDATA 637 



of simple Ascidians ; but the tailed larva shows some quite peculiar 

 features. The pharynx remains quite small, and there is an enormous 

 ectoderniic vesicle at the root of the tail which obviously serves for 

 flotation. In front of this there is a mesodermic mass, and behind it 

 are situated two mesodermic masses, and behind these again the 

 notochord with its longitudinal muscles. 



The nervous system is represented by a very large solid mass of 

 cells a brain-vesicle being apparently absent. The front mesodermic 

 masses give rise to the hoop-like muscles of the pharynx ; the atrial 

 cavity arises as a median invagination, forking below, and each fork 

 communicates with the pharynx by a series of undivided "proto- 

 stigmata." As the hoop-like muscles develop the great ectodermic 

 vesicle shrinks, and finally the embryo differs from the adult only in 

 carrying tail and notochord. These are finally absorbed and the 

 development is then complete. 



The Ascidiae luciae and the Thaliaceae agree in being purely 

 pelagic forms which pass the whole of their lives swimming in the 

 open sea. The tailed larva cannot, therefore, fix itself to any solid 

 substratum. To meet these altered conditions the development 

 undergoes three different modifications in Pyrosoma, Salpa, and 

 Doliolum respectively. 



In Pyrosoma the embryo is a mere sketch of the typical larval 

 form ; it begins to bud before it is half -developed, and founds the 

 colony round the remains of the yolky egg, after which the original 

 person disappears. In Salpa the egg must have originally developed 

 into a tailed larva, but this larva fixed itself to the maternal atrial 

 wall at the mouth of the oviduct, and there grew till it was ready to 

 break loose. Finally, in Doliolum, the larval organs, tail, and noto- 

 chord are retained till the adult form has been reached. 



ASEXUAL REPRODUCTION IN UKOCHORDA 



The asexual reproduction of the Urochorda is a most fascinating 

 subject, and has attracted an immense amount of research which is 

 embodied in an extensive literature. The whole subject, however, 

 has been reviewed in a most masterly way by Hjort (1896), whose views 

 have stood unchallenged for the last eighteen years. Ritter (1897), 

 the only worker who has since then published an important paper on 

 the subject, confirms Hjort in all important matters and endorses his 

 views. It would be waste of time, therefore, to trouble the reader 

 with the views of earlier workers whose ideas on the subject were by 

 no means clear. Pizon, however, both before (1891) and since Hjort's 

 paper appeared (1905), has made a special study of the extraordinary 

 condition of affairs prevailing in the family of compound Ascidians 

 termed Diplosomidae. 



According to Hjort, then, there are two main types of bud- 

 formation found amongst Urochorda, which, so far as we can see, 

 must have been independently evolved. These are pallial budding, 



