GIBSON— CEPHALOCJIOEDA : " AMPHIOXIDES." 217 



anatoiit. 



(1) The Larv^. 



External Characters. 



The great majority of specimens found arc, as already stated, A. pelagicus, between 

 Avhicli and A.valdwice there can be no confusion. None can be assigned to A. sternurus. 

 The two animals to which Goldsclimidt gave this name differ, however, so slightly 

 from A. pelagicus, that their specific distinctness may perliaps be doubted. Their 70 

 myotomes would not represent a variation outside the range which we know from 

 Andrews (15^93), Punnett (1903), and others to be of normal occurrence among the 

 Cephalochorda, although I found no A. pelagians possessing more than 68. 



Amphioxides pelagicus can at once be distinguished from A. valdicia; by the 

 conformation of its tail- and fin-membranes. The majority of my s^iecimens showed 

 the consti'iction at the anterior limit of the tail represented in fig. 2, but in a large 

 number, as in Goldschmidt's, it was absent. The sharp point and great relative length 

 of the tail, with its tapering notochord and myotomes, are characteristic. The dorsal 

 fin-membrane begins at the level of the 23rd-25th myotomes, the venti'al — on an 

 average — at the level of the 40th, but further forward in many of the older animals. 

 Between its anterior end and the gill-region, a curious regular crinkling of the ventral 

 edge of the body was often seen, suggesting at first sight rudimentary fin-ray boxes : 

 this suggestion was, of course, fallacious ; probably the appearance is simply an effect 

 of preservation. The fin-ray boxes, anteriorly, often appear in side view to be completely 

 covered by the myotomes, and at the hinder end of the first myotome are replaced by an 

 undivided, tapering canal. Their number per myotome ranges from 2 at the extreme 

 anterior end to 4 at the base of the tail. The pre-oral organ is large and oval in 

 contour. The mouth and adjoining organs are hard to see satisfactorily in whole 

 preparations ; a characteristic feature, usually seen, is a dark band passing obliquely 

 backwards below the posterior end of the mouth. The gill-bars appear in whole 

 preparations as roughly square, daik areas, hollowed ventrally by the coelomic canals, 

 in the floor of which the fibres of the branchial muscles are conspicuous. 



Amphioxides valdivice has a neater and more compact appearance. The tail is paddle- 

 shaped with a marked constriction at its base, shorter and broader than tiiat of 

 A. pelagicus. Tail, notochord, and myotomes all end bluntly. The dorsal fin-membrane 

 starts behind the gill-region — level with the 32nd-33rd myotome, the ventral level with 

 the 43rd — further back than in A. pelagicus. Eoth rise gradually from the body- 

 surface and are comparatively low. The fin-ray boxes are taller and thinner than in 

 A. pelagicus, and there are about 5 to a myotome throughout. Their extension to the 

 anterior end of the nervous system is very characteristic ; they everywhere project 

 above the myotomes. The pre-oral pit is small and compact, the mouth very long. 

 Through it can be seen the anterior gill-bars lying in the right wall of the pharynx ; 

 these have somewhat the appearance of figures of 8 : further back the folds become 



