408 ZOOLOGY. 
or ascidian in the same stage of growth. No 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, 2), 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 
o& Epigonichthys 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. 
Cuass II.—LEPTOCARDIL 
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 ciliaftd, Sree-swimming. 
A single order (Pharyngobranchi), family (Amphioxini), 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, 
Langerhans, Lankester, and Rice (Amer. Nat., 1880); also Willey, 1894, 
