CHAPTERS ON EVOLUTION. 



the digestive apparatus and heart), the larval ascidian becomes the 

 mature sea-squirt. 



It is of interest to note that in a few aberrant members of the 

 sea-squirt group the larval or immature characteristics are retained 

 throughout life. Such are the Appendicularians 

 (Fig. 90), which, although ranked as veritable sea- 

 squirts, retain, as a permanent belonging, the tail (/) 

 which their neighbours possess only in the days of 

 their youth. Within this permanent tail the noto- 

 chord (ri) appears developed as in the fleeting ap- 

 pendage of other sea- squirts, whilst the other organs 

 of sea-squirt existence (digestive system [j], heart 

 [<#], &c.) are fully developed. From the possession 

 of this notochord these curious animals appear as 

 unique invertebrates, and stand alone amongst their 

 fellows as presenting the closest resemblance to the 

 vertebrate animals. In the Appendicularians we 

 may perceive the existing representatives of the stock 

 and ancestry which gave origin alike to the fixed 

 sea-squirt race and to the great vertebrate group 

 itself. These " permanent larval forms," as Appen- 

 dicularia and its neighbours are termed, thus present 

 us with the least modified members of their class, 

 with the primitive and unchanged organism whose 

 development in other directions has produced the 

 APPEN'DICULARIA. highest races of living beings. Of these organisms 

 Darwin himself remarks that, "if we may rely on 

 embryology, ever the safest guide in classification, it seems that we 

 have at last gained a clue to the source whence the vertebrates 

 were derived. We should then be justified in believing that at an 



extremely remote period 

 a g rou P f animals ex- 

 isted, resembling in many 

 respects the larvae of our 

 present Ascidians, which 

 diverged into two great 

 branches the one retro- 

 grading in development 

 and producing the pre- 

 sent class of Ascidians, 

 the other rising to the 



crown and summit of the animal kingdom by giving birth to the 

 Vertebrata." 



Ascending now to the confines of the Vertebrate sub-kingdom 

 of animals, we may trace the development of the curious little fish 

 known as the Lancelet or Amphioxus (Fig. 91, b) a form interesting 



FIG. 91. AMPHIOXUS, OR LANCELET. 



