408 ZOOLOGY. 



or ascidian in the same stage of growth. JSTo vertebrate 

 features are yet developed. 



Soon the lively ciliated gastrula elongates, the alimentary 

 tu lie arises from the primitive gastrula-cavity, while the edges 

 of the llatteiied 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 laneelet at one time in its life presents Ascidian 

 features, yet as ]>alfour 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, jSTorth- 

 ern Australia, has been described by Peters under the name 

 of Epigoniclitliys nilh'llnx. 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 

 ; \vmtv-three millimetres in length. 



/ o 



CLASS II. LEPTOCARDII. 



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



9k(.'lttn ; notorhurd pirxixtint, no bruin; no cranium ; no paired fins ; 

 blooil cnl'irli'xs ; a metamorphosis ; gastrula ciliated, free-swimming. 



A Mngle order (Pharyngobranchi), family (Amphioxiui), and genus 

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



Laboratory Wurk. 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. Brooks has 

 found the young in the later free-swimming stages on the surface of 

 tli< ocean near Fortress Monroe. 



