LANCELETS. 



559 



tentacles, growing from the margin of the hood-like expansion of skin which 

 surrounds the mouth. The notochord extends to the anterior and posterior 

 extremities of the body, reaching beyond the muscle-plates, and likewise in advance 

 of the front extremity of the overlying nerve-chord ; the latter feature being 

 peculiar to the lancelet. An aperture distant about two-thirds of the whole length 

 from the head, and opening in the middle line of the lower surface of the body, is 

 the outlet of a large cavity, or atrial chamber, surrounding most of the internal 

 organs, and especially the large pharynx; and the vent, as in many tadpoles, is 

 situated high up on the left side, near the hinder end of the body. The reproduc- 

 tive organs, which form oval structures lying below the muscle-plates, differ from 

 those of the Vertebrates in that they consist of a large number of perfectly distinct 

 chambers, corresponding to the muscle-segments of the region of the body along 

 which they extend. In connection with the fins, it should be observed that, except 

 at its two extremities, the dorsal fin is supported by a series of gelatinous rays, each 

 lying in a chamber of its own ; while the ventral portion of the caudal fin has a paired 

 series of similar supports. In young and transparent examples, the pharynx, or that 

 portion of the alimentary tract immediately behind the mouth, is distinctly visible 

 through the walls of the body, and can be seen to be perforated on each side by a very 

 large number of vertical gill-slits, opening into the atrial chamber. In the living 

 creature an almost continuous current of water is drawn, for the purpose of breath- 

 ing and feeding, through the mouth into the pharynx, whence it escapes by means of 

 the gill-slits into the atrial chamber, from which it is discharged through the pore. 

 Unlike even the lowest Vertebrates, lancelets have no cartilaginous skull ; the only 

 solid structure in the head taking the form of a ring of cartilage in the hood surround- 

 ing the mouth, which gives off a series of processes for the support of the feelers. 

 Although paired eyes, as well as organs of hearing, are totally wanting in these 

 strange little creatures, a pigment-spot at the front end of the nerve-tube represents 

 a median eye ; behind which is a small nasal pit, communicating in the larva by 

 means of a small pore with the front of the nerve-tube. With regard to the other 

 soft- parts, it will suffice to mention that the anterior extremity of the nerve-tube 

 is not expanded to form a true brain ; and that the heart is represented merely by 

 a series of pulsating dilatations of the great blood-vessel ; the blood itself being 

 devoid of colour. 



Lancelets are represented by some eight or nine species, all of which may be 

 included in a single genus ; although one from the Bahamas is peculiar on account 

 of the unsymmetrical arrangement of its reproductive organs. Essentially littoral 

 forms, inhabiting shallow water, especially where the bottom is sandy, these 

 creatures have an almost universal distribution on the temperate and tropical 

 coasts, although they are often curiously local. The European form has been 

 recorded from Scandinavia, Heligoland, the English Channel, France, the Medi- 

 terranean, and Chesapeake Bay, growing to an unusual size in French waters. 

 Other species occur on the Atlantic and Pacific shores of North and South 

 America, as well as on the coasts of Australia, Japan, Ceylon, and the Fiji Islands. 

 Mr. A. Willey remarks that the lancelet " possesses an extraordinary capacity for 

 burrowing in the sand of the seashore or sea-bottom. If an individual be dropped 

 from the hand on to a mound of wet sand, which has just been dredged out of the 



