646 INVEETEBRATA CHAP. 



simple Ascidians once had the power of budding and lost it. The 

 study of the larvae of simple Ascidians does not bear out this view. 

 In the general disposition of their organs, and in the formation of 

 their stigmata, they are more primitive than the simplest compound 

 Ascidiau, Clavelina. If, then, the epicardia were not originally connected 

 with budding they may possibly represent anterior coelomic sacs 

 perhaps the collar-cavities since they lie externally to the other viscera. 



The origin of the pericardium as a single median evagination of 

 the pharynx is, however, hard to explain. The pericardium of the 

 higher Vertebrata is formed from a portion of the splanchnocoele 

 behind the branchial region, and this splanchnocoele is simply the 

 lower end of the front part of the conjoined trunk -cavities. Now the 

 trunk - cavities of the Urochorda are mainly represented by the 

 longitudinal muscles of the tail, and by the so-called anterior mesen- 

 chyme cells^ which form the mesoderm of the adult, and actually, for 

 some time, form part of the lateral walls of the embryonic gut. Yet 

 there can be little doubt that the pericardium and heart of Urochorda 

 are homologous with those of higher vertebrates ; they are situated 

 in the same position in the adult, and the heart, by its shape, reminds 

 one of the S-shaped heart of the higher vertebrate embryo. 



Perhaps the only suggestion which can be offered is this. The 

 pericardium is held back in development ; it does not develop until 

 the later part of larval life, and its origin from the pharyngeal wall 

 may represent the outgrowth of the trunk -cavities from the archenteric 

 wall of Amphioxus. We may suppose that the front portions of 

 these cavities develop late, and independently of the hinder portions, 

 just as we explain the larval and adult mouths of Echinodermata as 

 the two parts of an ancestral mouth, one of which is held back in 

 development. A detailed study of the development of Larvaceae 

 would probably throw Light on this question. 



In any case there can be no doubt that the ancestor represented 

 by the Ascidian tadpole, and in certain degree by the adult in the 

 group Larvaceae, possessed a well-developed brain-vesicle with hemi- 

 spherical visual optic lobes, a hypophysis or pituitary body opening 

 into the stomodaeum, also a well-developed tail, and a definite heart ; 

 and that in all these respects it was far in advance of the stage repre- 

 sented by Amphioxus. The distance which separates the points at 

 which Amphioxus and Urochorda diverged from the main stem of 

 Vertebrata, is almost comparable to that which separates the points 

 of origin of Amphioxus and Enteropneusta from the same stem. 



[Note. It is necessary that this group of animals, lying, as it does, 

 on the borderland of both Invertebrata and Vertebrata, should be 

 treated by the authors of both Vol. I. and Vol. II. ; and, as each author 

 is solely responsible for the facts and opinions contained in his book, 

 the reader must be prepared to find that the views adopted by each 

 author, especially regarding this overlapping group, may not wholly 

 coincide. EDITOR.] 



