T.KPTDCAliDIANS. 



1211 



hiive gained an insight into the strncture and de\elop- I Tlie tow forms of the order belong to a single 



nient of the Leptocardians. | family, the 



Fa I AMPHIOXlDiE 



whose characters thus coincide with those of the ordt'r. 

 NOr have reasons been discovered for the estal)lishment 



within this tumily of more than one genus. 



Cem s BRANCHIOSTOMA. 



Body more or less elongated, lanceolate, pointed and lateralhj compressed at both ends, intermediatehj trianf/ulur 

 in section. Mouth ventral, elliptical, fringed with tentacles, hut jawless. A dermal fold edges the hodg from 

 the mouth round the tip of the snout, along the hnck and round the tip of the tail, and, usually'', forward along 

 the rentral margin to the abdominal pore, whence two lateral folds extend to the mouth. Anus situated far back, 



beside the ventral margin. 



This genus is probal)ly 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 luimbers. Usu- 

 ally it lives on sandbanks, where it has shelter and 

 plenty of food. Its diet apipears to consist exclusively 

 of Infusoria and similar minute creatures or the lowest 

 vegetable forms. If a Branchiostoma (PI. LIII, tig. (i) 

 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 liottom. 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 al)ove, its almost 

 entire lack of that we call head. With the same justice 

 IS 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, magnifiecl. 

 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 vertebrie; /, cartilaginous ring, supporting the external mouth aper- 

 ture; ij. cirri; ^f!/, spinal cord (^>nyelo7i); Ch, notochord (chorda dor- 

 salis). .\fter 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 



" Amphioxini, .J. Ml'llek. I. c. Afterwards the family was called Braiwhiostoinidtn in Bonaparte, C'«^ .Mtt. Pesci Europ. (184(5). 

 9 and 92. 



'' Tn the Australian Branchiostoma {Ejngonichthys) culiellus described by Peters this part of the dermal fo]<l is said to l)e rudimentary, 

 anus of this species being accordingly situated in the median ventral line. 



