PRIMITIVE VERTEBRATES 



301 



some species suggests the term " acorn-headed ". The mouth 

 is situated on the under side, at the base of this proboscis. Next 

 comes a comparatively short region, named from its appearance 

 the collar, while the rest, and by far the longest part, of the 

 animal may be termed the trunk. On the upper side of the 

 trunk, behind the collar, are a con- 

 siderable number of gill-slits ar- 

 ranged in pairs, and forming the 

 external apertures of gill-pouches 

 which communicate internally with 

 the digestive tube. They resemble 

 in many respects the corresponding 

 structures in the Lancelet. A small 

 notochord has also been identified, 

 but here we have the opposite 

 extreme from what is found in an 

 Ascidian tadpole, for the structure 

 in question is a small rod which 

 projects into and supports the base 

 of the proboscis. It is in reality 

 a thickened forward outgrowth 

 from the digestive tube, and has 

 a peculiar microscopic structure 

 which is distinctive of notochords 

 wherever they are found. The fact 

 that it grows out of the gut is also 

 a point in support of its notochordal 

 nature, for in more typical cases, as, e.g., Lancelet or Frog, the 

 notochord arises as a thickening in the wall of the digestive 

 tube. The remaining test of a Vertebrate is also answered in 

 a fairly satisfactory way, for what may be described as the 

 central nervous system in this creature is a more or less hollow 

 thickening running along the dorsal part of the body in the 

 collar region. 



There are certain other more doubtful claimants to a place 

 in the Hemichorda, but a discussion of their characters would be 

 out of place in a preliminary sketch of the animal kingdom. 



Fig. 175. — Acorn-headed Worm [^Balanoglossus , 



