14 /OOLOGY SECT. 



rfseiublancos in scmic |H;>int.s to a jihj'luiii — that rif the Echino- 

 (lerinata — wliich it has l)e<'ii the eiistoiii te ])kxce very low 

 (l(.\vii in tlie invertehrate series. 'I'lie tornaria larva of Balano- 

 L^lossns exhibits a striking likeness to an e<'hinop:edinni (Vol. I., 

 ]i. 432), and, though this likeness between the larv;c does not estab- 

 lisli near connection, it .suggests, at least, that an alliance exists. 

 Between actinotrocha, the larva of Phoronis (Vol. I., p. 358) and 

 tornaria there are some striking p(3ints of resemblance ; and a 

 pair of gastric diverticula in the former have sometimes been 

 compared with the single notochord or oesophageal diverticulum 

 of the Hemichorda. 



SUB-PHYLUM AND CLASS II.— UEOCHORDA. 



The Glass Urochorda or Tunicata comprises the Ascidians or 

 Sea-Squirts, which are familiar objects on every rocky sea-margin, 

 together with a number of allied forms, the Salp;e and others, all 

 marine and for the most part pelagic. The Urochorda are specially 

 interesting because of the remarkable series of changes which they 

 undergo in the course of their life-history. Some present us with 

 as marked an alternation of generations as exists among so 

 many lower forms ; and in most there is a rdroyressive inda- 

 morpliosis almost, if not quite, as striking as that which has been 

 described among the parasitic Copepoda or the Cirripedia. In by 

 flrr the greater number of cases it would be quite impossible by 

 the study of the adult animal alone to guess at its relationship 

 with the Chordata ; its affinities with that phjdum are only de- 

 tected when the life-history is followed out, the notochord and 

 other higher structures becoming lost in the later stages of the 

 metamorphosis. Multiplication by bndding, so common in the 

 lower groups of Invertebrata, but exceptional or absent in the 

 higher, is of very general occurrence in the Urochorda. 



1. Example of the Class— The Ascidl\n or Sea-Squirt. 



{Ascidia.) 



Sea-squirts are familiar objects on rocky sea-shores, where they 

 occur, often in large associations, adhering firmly to the surface of 

 the rock. When touched the Ascidian ejects with considerable 

 force two fine jets of sea-water, which are found to proceed from 

 two apertures on its upper end. The shape of the Ascidian, 

 however, can only be profitably studied in the case of specimens 

 that are completely immersed in the sea-water, specimens not 

 so immersed always rmdergoing contraction. In an uncontracted 

 specimen (Fig. 715), the general shape is that of a short cylinder 

 with a broad base by which it is fixed to the rock. The free end 

 presents a large rounded aperture, and some little distance from it 



