FOURTH SUB-CLASS—LEPTOCARDII.
Skeleton membrano-cartilaginous and notochordal, ribless. No brain. Pulsating sinuses in place of a heart. Blood colourless. Respiratory cavity confluent with the abdominal cavity; branchial clefts in great number, the water being expelled by an opening in front of the vent. Jaws none.
This sub-class is represented by a single family (Cirrostomi) and by a single genus (Branchiostoma);[48] 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 Hæckel, 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” (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 Epigionichthys pulchellus; it was found in Moreton Bay.