'IKE LANCELET. 147 



DIVISION IV. LEPTOCARDII * (FISHES WITH THIN HEARTS). ORDER 



PHARYNGOBRANCHII. 



This concluding group of the fish class contains only two species 



FAMILY CIRROSTOMI. THE LANCELET. t 



This little fish is never more than three inches long, is transparent and iridescent, is very 

 active, and has a fin extending from near the snout round the tail to the vent. The body is com- 

 pressed, the head pointed ; the eye is a dark speck in a slight depression of the skin ; the mouth 

 is an elongated oval, placed longitudinally and margined by slender filaments, which are ciliated and 

 supported by a cartilaginous framework, which extends round the mouth. Behind the mouth is the 

 pharynx, which is perforated by numerous vertical or slightly oblique branchial clefts, which extend 

 far down the length of the body. Behind the pharynx is a simple stomach prolonged into a 

 straight intestine, which terminates in a vent near the root of the tail. There is another aperture in 

 front of the vent, which opens into a cavity distinct from the pharynx, which extends forward towards 

 the mouth. The function of this pore appears to be to carry off the water, which, propelled inward 

 to the pharynx by the mouth, passes through the branchial slits into the external cavity. In an 

 early stage of development the branchial slits are entirely exposed on the sides of the body. There 

 are no gills. The liver is an appendage to the intestine. The reproductive organs are glandular 

 masses, arranged in a row along each wall of the body cavity. The heart has no trace of the 

 muscular character seen in the higher Vertebrata, which have hence been distinguished from the 

 Amphioxus as Pachycarclii, while the name Leptocardii indicates the thin wall which is here seen 

 in that organ. In fact, the heart is no more developed than is the heart of a chick when it first 

 appears in the first few days of incubation. Contractions take place only at the rate of about one 

 a minute. All the principal blood-vessels are contractile. The blood is quite colourless, and, as 

 in the lower animals and the young 

 of Vertebrata, the blood-corpuscles are 

 nucleated, so that the red corpuscles 

 have not as yet been formed. The LAXCELET. 



skeleton is very imperfectly developed, 



and, beyond that part ah'eady referred to, around the mouth is limited to a notochord, which shows no 

 trace of transverse division into vertebra? or of superior or inferior arches or ribs. It extends some dis- 

 tance in front of the spinal cord. There is therefore no skull or brain in any ordinary sense of the 

 term, and the anterior extremity of the spinal cord, instead of enlarging, diminishes in size ; it 

 gives off nerves to the eye and the filaments round the oral region. The fish displays many analogies 

 with the invertebrate group termed Ascidians, and somewhat resembles amphibians in the mode 

 of formation of the cavity external to the branchial slits. The surface of the body is smooth and 

 entirely destitute of scales. When first studied the Lancelet was mistaken for a slug, just as the 

 Hag was mistaken for a worm. It has been kept in captivity, and observed to usually bury itself 

 a little in the sand when disturbed. It is extremely sensitive to light ; it often lies as though dead 

 for half an hour or an hour together. This fish can scarcely claim to belong to the Vertebrata ; 

 it wants many of the more striking characteristics of fishes, and certain observers have sometimes 

 surmised that it may possibly be an embryonic fish of which the mature form is unknown. This, 

 however, is unlikely. But it differs from other fishes in characters in which they all agree, and 

 differs also from fishes in points of structure which are common to them and higher Vertebrates. It 

 might well form the type of a class standing alone, and helping by its low grade of organisation 

 to indicate one of the lost steps of continuity between vertebrate and invertebrate animals. It 

 frequents shallow water, and is widely distributed in temperate and tropical seas. A second species 

 has been found in Moreton Bay. 



FOSSIL FISHES. 



A large proportion of fossil fishes belong to the division Palaeichthyes. This group comprises 

 most of the fishes which have been met with in the Primary rocks and many of those found in the 



* Leptos, thin ; cardia, heart. f Amphioxus lanceolaius. 



