OF FISHES. 227 



with the ideal Vertebrate, (fig. 63,) when both are contrasted 

 with the more complicated, warm-blooded quadruped (fig. 134). 



Amphioxus. The most lowly organized of all known Ver- 

 tebrates is a fish which is commonly called the Lancelot, and 

 sometimes the Sand-Eel, on account of its habit of burrowing 

 in the sand of the sea-shore. It is so remarkably transparent 

 that its whole internal organization can be seen with a good 

 microscope. It has no external appendages excepting a circle 

 of feelers (fig. 133, be) about the mouth, (j,) and therefore, for 

 the lack of fins, the whole duty of locomotion devolves upon the 

 highly muscular, lance -like tail. The head (f) is peculiarly 

 adapted, by its sharp, thin front, for the purpose of penetrating 

 the compact sand-beach. The mouth (j) is an elongate open- 

 ing situated on the under side of the head, at a considerable dis- 

 tance behind its front. The feelers (be) which surround it are 

 largely supplied with nerves, and therefore in all probability are 

 highly sensitive organs of touch, and serve as efficient means 

 for obtaining food. 



Between the mouth and the entrance (g l ) to the throat, there 

 is considerable space, the buccal cavity, within which certain 

 oval bodies (I, II) project from above like a pair of palates. 

 The latter are covered by constantly vibrating threads, cilia, 

 which keep up a current of water from the mouth toward the 

 gills, (b to b 1 ,) and at the same time furnish the means of floating 

 fine particles of food into the throat. The entrance to the latter 

 is a moderate aperture, (g\) but the throat itself is a very large 

 cavity, which performs at the same time the office of a breathing 

 apparatus, or gill-chamber. Its sides are perforated by several 

 parallel slits, (bo,) which extend from its upper (b) to its lower 

 (6 1 ) margins, through which the water pours, as between the 



dorsal artery ; h 5 , the abdominal vessel ; 6, the upper, and b 1 , the lower point 

 of junction of the branchial vessels (br) with the dorsal (A*, 7*6) and ventral 

 (7*1, h 5 ) vessels ; ac, abdominal cavity ; ap, abdominal pore ; nrl, the anterior, and 

 nr, the posterior end of the main nerve or spinal marrow ; ns, sheath of nr, nr 1 ; 

 o, the eye; n, the olfactory nerve; nv, the facial nerves; ov, the reproductive 

 organ. From Owen. 



