BALANOGLOSSUS. 251 



of a species of Balanoglossus was demonstrated by ELIAS 

 METSCHNIKOFF. Shortly afterwards, Metschnikoff's dis- 

 covery was confirmed and amplified by ALEXANDER 

 AGASSIZ. 



The superficial likeness between Tornaria and such Echi- 

 noderm larvae as Bipinnaria or Auricularia is astonishing, 

 and a renewed study of the detailed organisation of 

 Tornaria, recently made by MORGAN, appears to have 

 established the fact, originally insisted upon by Metschni- 

 koff, that this resemblance can only be accounted for on 

 the ground of genetic affinity. 



In Figs. 119 and 120 two types of larvae, Tornaria 

 and Auricularia, are shown side by side ; and although 

 unfortunately they are not figured from exactly the same 

 aspect, yet it is obvious at a glance that, in spite of certain 

 differences which will be enumerated below, they both 

 belong to the same category of larval forms. 



A highly characteristic feature of these larvae is the 

 remarkable ectodermal ciliated band which constitutes a 

 perfectly symmetrical but somewhat complicated undulat- 

 ing seam round the body. The larvae are strictly pelagic, 

 and swim about in the open sea by means of their cilia ; 

 but the latter, instead of being distributed evenly over the 

 whole surface of the body, are concentrated in the region 

 of the ciliated bands which are composed of thickened 

 ectoderm. 



In Tornaria there are two ciliated bands, viz.: i) the 

 above-mentioned undulating seam which is usually known 

 as the circnmoral or longitudinal ciliated band, and 2) a 

 pastoral circular ciliated band. Only the former is present 

 in Auricularia, and the absence of the circular band in this 



form constitutes one of the chief differences between the 



* 



two larvae. 



