78 ORDER LEPTOCARDII. 



if a Hag be placed in a pitcher of sea-water, it will speedily 

 convert this into a semi-transparent jelly ; and that, if placed in 

 a fresh quantity of water, it will change this in the same manner. 



ORDER V. LEPTOCARDII. , 



642. The most imperfectly formed of all Fishes is undoubt- 

 edly the Amphioxus, or Lancelot, which of itself forms the last 

 order of Fishes. It has even by many naturalists been removed 

 altogether from the Vertebrated sub-kingdom, on account of the 

 almost complete absence of what are usually regarded as the 

 distinguishing peculiarities of that group. Nevertheless, an at- 

 tentive examination of its structure shows, that it bears a closer 

 resemblance to the true Fishes than to any other animals ex- 

 hibiting their conformation in (as it were) a degraded form. It 

 is of very diminutive size, scarcely an inch in length, very 

 slender, and almost transparent. The body is compressed later- 

 ally ; and there are no pectoral, ventral, anal, or caudal fins, but 

 only a narrow membranous border which runs along the whole 

 of the dorsal and a part of the ventral surface, and expands at the 

 tail into a lancet-shaped fin. There are no eyes, nor any vestige 

 of any external organ, except the mouth, which is surrounded by 

 very small cartilaginous points. There is scarcely any trace of 

 a vertebral column, or of ribs ; 'yet the muscles are arranged 

 with great regularity, on the plan of those of Fishes in general. 

 The mouth leads into a large branchial cavity, in which currents 

 are produced in the water by the action of cilia ; the entrance to 

 the intestinal canal is at the back of this cavity, and the food is 

 no doubt conducted to it by ciliary action. One of the most 

 curious parts of its structure is the complete absence of cerebral 

 hemispheres, and even of ganglia of special sense ; the spinal 

 cord being, apparently, the only centre of its nervous system. 

 Thus it may be characterised as one of those " experiments pre- 

 pared for us by Nature;" exhibiting to us a case, in which the 

 Cerebrum is never developed ; the phenomena of which closely 



