LEPTOCARDIANS. 
1211 
have gained an insight into the structure and develop- The few forms of the order belong to a single 
ment of the Lept'ocardians. ' family, the 
Fam. AIPHI0XI1) 
whose characters thus coincide with those of the order. 
Nor have reasons been discovered for the establishment 
within this family of more than one genus. 
Genus BRANCHIOSTOMA. 
Body more or less elongated , lanceolate , pointed and laterally compressed at loth ends , intermediately triangular 
in section. Mouth ventral, elliptical, fringed with tentacles, hut jawless. A dermal fold edges the body from 
the mouth round the tip of the snout, along the lack and round the tip of the tail, and, usually b , forward along 
the ventral margin to the abdominal pore, whence two lateral folds extend to the mouth. Anus situated far lack, 
beside the ventral margin. 
This genus is probably dispersed throughout the 
temperate and tropical coasts of all the oceans, but 
sporadically. It is known from Europe, Australia, Peru, 
Brazil, and the east coast of North America. Where 
it occurs, it is generally met with in numbers. Usu- 
ally it lives on sandbanks, where it has shelter and 
plenty of food. Its diet appears to consist exclusively 
of Infusoria and similar minute creatures or the lowest 
vegetable forms. If a Branchiostoma (PI. LIII, fig. 6) 
be placed in a vessel of seawater with a little sand on 
the bottom, it will perhaps lie still on its side for a 
long while, as if dead; but on being touched it leaps 
up and tosses itself to and fro with swaying move- 
ments, until it eventually lies down to rest again on 
the sand or buries itself in the bottom. This is done 
with great rapidity, in a moment; and afterwards it 
only sticks the oral end up into the water, opens its 
wreath of tentacles, and commences in its mouth cavity 
the ciliary motion that conducts water and food into the 
respiratory cavity, which also serves as an oesophagus. 
That which first gives this animal, in contradis- 
tinction to the rest of the vertebrates, a singular posi- 
tion in the system, is, as mentioned above, its almost 
entire lack of that we call head. With the same justice 
as we call a mussel a headless mollusk, we may also 
describe Branchiostoma as a headless vertebrate. 
Fig. 365. Anterior end of the body of the Lancelet, seen from the 
right, magnified. 
a, site of the olfactory organ (situated on the left side); b, site of 
the eye (pigment spot); c, second pair of nerves, counting from in 
front; d, spinal nerves; e, homologues of the upper spinous processes 
of vertebras ; /, cartilaginous ring, supporting the external mouth aper- 
ture; g, cirri; My, spinal cord ( myelon ); Ch, notochord (chorda dor- 
salis). After Quatrefages and Huxley. 
Throughout the length of the body — excepting 
the extreme tips of the snout and tail — there run in 
Branchiostoma (fig. 365) a spinal cord (My) and a 
notochord (Ch), the latter representing a structure pre- 
sent in the embryos of all other vertebrates. But in 
them the notochord terminates anteriorly under the 
swelling of the medullispinal canal, the rudimentary 
a Amphioxini, J. Muller, 1. c. Afterwards the family was called Branchiostomidce in Bonaparte, Cat. Met. Pesci Europ. (1846), 
pp. 9 and 92. 
6 In the Australian Branchiostoma ( Epigonichthys ) cidtellus described by Peters this part of the dermal fold is said to be rudimentary, 
the anus of this species being accordingly situated in the median ventral line. 
