viii PREFACE. 



progressive development some of these types have come 

 to so closely resemble, superficially, certain of the larger 

 groups of Invertebrates, such as the Molluscs and Worms, 

 that it is only at a comparatively recent date they have 

 found their way out of these groups into the Protochor- 

 data. Many of these misleading resemblances are now 

 interpreted as parallels of structure springing from parallels 

 in life habit, seen not only in the general body form, but 

 in special organs, such as the breathing apparatus of the 

 Ascidians and Molluscs. 



By the side of parallelisms are real invertebrate and 

 vertebrate affinities ; so that the problem of resolving 

 these various cases of original and acquired likeness in 

 their bearing upon descent has become one of the most 

 fascinating which modern Zoology affords. For example, 

 among the real invertebrate ties of the Protochordates are 

 the ciliated embryos of Balanoglossus and Amphioxus, 

 the Tornaria larva and ciliated ectoderm of Balanoglossus. 

 The nervous system of Balanoglossus presents both ver- 

 tebrate and invertebrate characters ; the respiratory sys- 

 tem is identical with that of Amphioxus, while in the 

 embryonic development there are many resemblances inter 

 se. In short, in Balanoglossus and the Ascidians the 

 invertebrate type of structure, whether original or ac- 

 quired, predominates. But in Amphioxus the balance is 

 far on the other or vertebrate side of the scale, and this, 

 with its resemblances to lower forms, gives us the con- 



