2936 THE LANCE LETS 



provided with a continuous dorsal fin, expanded posteriorly into a caudal fin, and 

 continued forward to join the ring of feelers, or tentacles, growing from the margin 

 of the hood-like expansion of skin which surrounds the mouth. The notochord ex- 

 tends 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, sur- 

 rounding 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 reproductive 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 por- 

 tion of the caudal fin has a paired series of similar supports. In young and trans- 

 parent 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, open- 

 ing into the atrial chamber. In the living creature an almost continuous current of 

 water is drawn, for the purpose of breathing 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 surrounding 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 color. 



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 crea- 

 tures have an almost universal distribution on the temperate and tropical coasts, al- 

 though they are often curiously local. The European form has been recorded from 

 Scandinavia, Heligoland, the English Channel, France, the Mediterranean, 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 



