xiii PHYLUM CHORDATA 11 



Affinities. The inclusion of the Adelochorda in the phylum 

 Chordata is an arrangement the propriety of which is not uni- 

 versally 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 Ch'or- 

 dates, they are at least a greatly modified branch, taking its origin 

 from the base of the Chorcfate tree. The presence of the pre- 

 sumed rudimentary representative 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 Adelochorda are primitive Chordates 

 the fact is of special interest that among lower forms 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 Balano- 

 glossus exhibits a striking likeness to an Echinopsedium (vol. i. 

 p. J96A and, though this likeness between the larvse does not 

 establish a near connection, it suggests, at least, that an alliance 

 exists. Between Actinotrocha, the larva of Phoronis (vol. i. p. 330) 

 and Tornaria there are some striking points of resemblance ; and 

 the discovery in the former of a pair of diverticula resembling 

 the " notochord " of the Adelochorda lends support to the view 

 that Phoronis is nearly related to the present group. 



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 Salpse 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 meta- 

 morphosis 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 Chordata ; its affinities with that phylum 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 budding, so common in the 

 lower groups of Invertebrata, but exceptional or absent in the 

 higher, is of very general occurrence in the Urochorda. 



