408 ZOOLOGY. 



or ascidian in the same stage of growth. N"o vertebrate 

 features are yet developed. 



Soon the lively ciliated gastrula elongates, the alimentary 

 tube arises from the primitive gastrula-cavity, while the edges 

 of the flattened side of the body grow up as ridges which 

 afterwards, as in all vertebrate embryos, grow over and en- 

 close the spinal cord. When the germ is twenty-four hours 

 old it assumes the form of a ciliated flattened cylinder, and 

 now resembles an Ascidian embryo (Fig. 138, B), there 

 being a nerve-cavity, with an external opening, which after- 

 wards closes. The notochord appears at this time. 



In the next stage observed the adult characters had ap- 

 peared, the mouth is formed, the first pair of gill-openings 

 are seen, eleven additional pairs appearing. It thus appears 

 that while the lancelet at one time in its life presents Ascidian 

 features, yet as Balfour states " all the modes of develop- 

 ment found in the higher Vertebrates are to be looked upon 

 as modifications of that of Amphioxus." 



A second form of this group, from Moreton Bay, North- 

 ern Australia, has been described by Peters under the name 

 of Epigonichtliys cultellus. It differs from Amphioxus in 

 the presence of a high dorsal fin, in the want of a distinct 

 caudal and anal fin, with some differences in the structure 

 of the mouth and oral tentacles. It is from thirteen to 

 twenty-three millimetres in length. 



CLASS II. LEPTOCARDII. 



Comprising the lowest Vertebrate known ; body lancet-shaped, with no 

 skeleton ; notochord persistent, no brain ; no cranium ; no paired fins ; 

 blood colorless ; a metamorphosis ; gastrula ciliated, free-swimming. 



A single order (Pharyngobranchi), family (Ampbioxini), and genus 

 (Amphioxus), each with the characters of the class. 



Laboratory Work. The structure of the lancelet can only be imper- 

 fectly made out by a triplet lens and higher powers ; but by sections 

 stained with carmine the anatomy can be well studied. 



LITERATURE. The writings of Kowalevsky, Stieda, Hatschek, 

 Laiigerhans, Lankester, and Rice (Amer. Nat., 1880); also Willey, 1894. 



