G1(^ 



FOURTH SUB-CLASS— LEPTOCARDII. 



Skeleton memlrano-cartilaginous and notocliordal, ribless. 

 JVo brain. Pulsating sinuses in place of a heart. Blood 

 colourless. Hes-piratory cavity confluent xoitli the abdominal 

 cavity ; hranchial clefts in great nuiiiber, the water heing ex- 

 pelled by an opening in front of the vent. Jaws none. 



Tliis sub-class is represented by a single family {Cirro- 

 stomi) and by a single genus {BrancMostoma) ■,^ it is the lowest 

 in the scale of fishes, and lacks so many characteristics, not 

 only of this class, but of the vertebrata generally, that Hpeckel, 

 with good reason, separates it into a separate class, that of 

 Acrania. The various parts of its organisation have been 

 duly noticed in the first part of this work. 



The " Lancelet " {BrancMostoma lanceolatum, see Fig. 28, 

 p. 63), seems to be almost cosmopolitan within the temperate 

 and tropical zones. Its small size, its transparency, and the 

 rapidity with which it is alile to bury itself in the sand, 

 are the causes why it escapes so readily observation, even at 

 localities where it is known to be common. Shallow, sandy 

 parts of the coasts seem to be the places on which it may 

 be looked for. It has been found on many localities of the 

 British, and generally European coasts, in North America, 

 the West Indies, Brazil, Peru, Tasmania, Australia, and 

 Borneo. It rarely exceeds a length of three inches. A 

 smaller species, in which the dorsal fringe is distinctly higher 

 and rayed, and in which the caudal fringe is absent, has been 

 described under the name of Bp)igioniehthys piidehellus ; it 

 was found in Moreton Bay. 



^ Tliis name is two years older than Amphioxus. 



