FOUETH SUB-CLASS— LEPTOCAEDII. 



Skeleton memhrano-cartilaginous and notochordal, ribless. 

 '^] No hrain. Pulsating sinuses in ^lace of a heart. Blood 

 colourless. Bespiratm'y cavity confluent with the abdominal 

 cavity ; branchial clefts in great number, the water being ex- 

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



This sub-class is represented by a single family (Cirro- 

 stomi) and by a single genus (Branchiostoma) -^ 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 Hseckel, 

 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 " Lancelot " {Branchiostoma 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 able 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 Epigjonichthys pulchellus; it 

 was found in Moreton Bay. 



^ This name is two years older than Amphioxiis. 



