256 THE PROTOCHORDATA. 



a great number of Invertebrate larvae, and is especially 

 characteristic of the free-swimming larvae (Trochophores, 

 or Trochospheres) of Annelids and Molluscs. We shall 

 return to this later. 



In Tornaria a single contractile cord passes from the 

 apical plate to the anterior enterocoel. 



There is no apical plate in Auricularia, nor in most of 

 the other Echinoderm larvae ; but there is reason to sup- 

 pose that it has been secondarily lost, since a transitory 

 ectodermal thickening at the apical pole can frequently 

 be observed in the course of their development ; and, 

 moreover, in what is probably the most primitive Echino- 

 derm larva known (viz. the larva of the Crinoid, Autedon), 

 there is a well-developed apical plate. 



Metamorphosis of Tornaria. 



The metamorphosis of Tornaria, as originally described 

 by Alexander Agassiz, takes place with relative sudden- 

 ness. According to the more recent account of the meta- 

 morphosis given by Morgan, a marked diminution in size 

 occurs ; the internal organs are drawn together in such a 

 way that the larval oesophagus, with the gill-pouches (see 

 Fig. 120), is drawn backwards into the body, and the 

 anterior enterocoel, as already described, is carried for- 

 wards into the prseoral lobe. The longitudinal (circum- 

 oral) ciliated band, which was the iirst to develop, is also 

 the first to disappear, while the posterior circular band 

 persists to a somewhat later stage. 



The Nciiicrtincs. 



It is thus evident that Balanoglossus, especially through 

 its Tornaria larva, shows undoubted marks of affinity to 



