xni PHYLUM CHORDATA 13 



Cephalodiscus, of which there are three sub-genera with fifteen 

 species, has been found at various widely separated localities in the 

 Southern Hemisphere (Straits >of Magellan, Borneo, Celebes, the 

 Antarctic) : species occur also off the coast of Japan and Korea. Some 

 live in shallow water : none have been found at a greater depth than 

 about 300 fathoms. Ehabdopleura has been found at moderate 

 depths in Norway, Shetland, the North Atlantic, France, the Azores, 

 Tristan d'Acunha, Celebes, and South Australia. It seems doubtful 

 if more than one species occurs. 



Affinities. The inclusion of the Hemichorda in the phylum 

 Chordata is an arrangement the propriety of which is not universally 

 admitted, and is carried out here partly to obviate the inconvenience 

 of erecting the class into a separate phylum. On the whole, however, 

 there seems to be sufficient evidence for the view that, if not the 

 existing representatives of ancestral Chordates, they are at least 

 a greatly modified branch, taking its origin from the base of the 

 chordate tree. The presence of the presumed rudimentary repre- 

 sentative of a notochord and of the gill-slits seems to point in this 

 direction. It should, however, be stated that, by some of those 

 zoologists by whom the members of this group have been most 

 closely studied, their chordate affinities are altogether denied. If 

 the Hemichorda are primitive Chordates, the fact is of special 

 interest that they show remarkable resemblances in some points 

 to a phylum that of the Echinodermata which it has been the 

 custom to place very low down in the invertebrate series. The 

 tornaria larva of Balanoglossus exhibits a striking likeness to an 

 echinopsedium (Vol. I., p. 422), and, though this likeness between 

 the larvae does not establish near connection, it suggests, at least, 

 that an alliance exists. Between actinotrocha, the larva of Phoronis 

 (Vol. I., p. 351) and tornaria there are some striking points 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.-UROCHORDA. 



The Class 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 Salpae 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 retrogressive metamorphosis 

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

 among the parasitic Copepoda or the Cirripedia. In by far 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 



