408 ZOOLOGY. 



or ascidian in the same stage of growth. Xo rertebrate 

 features are yet developed. 



Soon the Hvely ciliated gastrula elongates, the alimentary 

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

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

 afterwards, as in aU 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. 13S, B), there 

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

 wards closes. The notochord appears at this time. 



In the next stage oljserved the adult characters had. ap- 

 peared, the mouth is formed, the first pair of giU-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 ^loreton Bay, Xorth- 

 ern Australia, has been described by Peters under the name 

 of Epiuonichthys culteUus. 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 

 twentv-tliree millimetres in length. 



Class IL— LEPTOCARDII. 



CompTim,ng the loireM Vtrtebrate known ; body lancet-shaped, inth no 

 fl:i:ietnii ; nntorhord perdstent, nobraiii; no crmtivm ; no paired fm ; 

 blood rnl'.fhxs , n metainorphoisU ; gastrula cUuited, free-swimming. 



A sing^le order (Pbaryngobranclii), family (Ampliioxini), and genus 

 (Ampbioxus), eacli with the characters of the class. 



Laboratory Wiyi'k. — The structtire of the lancelet can onlr be imper- 

 ft'ctly made nut by a triplet lens and higher powers ; but by sections 

 siained with carmine the anatomy can be well studied. Brooks has 

 found the youn^ in the later free-swimming stages on the surface of 

 th' ocean near Fortress Monroe. 



