[CHAPTER XXVII]
THE LEPTOCARDII, OR LANCELETS
The Lancelet.—The lancelet is a vertebrate reduced to its very lowest terms. The essential organs of vertebrate life are there, but each one in its simplest form unspecialized and with structure and function feebly differentiated. The skeleton consists of a cartilaginous notochord inclosed in a membranous sheath. There is no skull. No limbs, no conspicuous processes, and no vertebræ are present. The heart is simply a long contractile tube, hence the name Leptocardii (from λεπτός, slender; καρδία, heart). The blood is colorless. There is a hepatic portal circulation. There is no brain, the spinal cord tapering in front as behind. The water for respiration passes through very many gill-slits from the pharynx into the atrium, from which it is excluded through the atripore in front of the vent. A large chamber, called the atrium, extends almost the length of the body along the ventral and lateral regions. It communicates with the pharynx through the gill-slits and with the exterior through a small opening in front of the vent, the atripore. The atrium is not found in forms above the lancelets.
The reproductive organs consist of a series of pairs of segmentally arranged gonads. The excretory organs consist of a series of tubules in the region of the pharynx, connecting the body-cavity with the atrium. The mouth is a lengthwise slit without jaws, and on either side is a row of fringes. From this feature comes the name Cirrostomi, from cirrus, a fringe of hair, and στόμα, mouth. The body is lanceolate in form, sharp at either end. From this fact arises a third name, Amphioxus, from ἀμφί, both; ὀξύς, sharp. Dorsal and anal fins are developed as folds of the skin supported by very slender rays. There are no other fins. The alimentary canal is straight, and is differentiated into pharynx and intestine; the liver is a blind sac arising from the anterior end of the intestine. A pigment spot in the wall of the spinal cord has been interpreted as an eye. Above the snout is a supposed olfactory pit which some have thought to be connected with the pineal structure. The muscular impressions along the sides are very distinct and it is chiefly by means of the variation in numbers of these that the species can be distinguished. Thus in the common lancelet of Europe, Branchiostoma lanceolatum, the muscular bands are 35+14+12=61. In the common species of the Eastern coasts of America, Branchiostoma caribæum, these are 35+14+9=58, while in the California lancelet, Branchiostoma californiense, these are 44+16+9=69.
Habits of Lancelets.—Lancelets are slender translucent worm-like creatures, varying from half an inch (Asymmetron lucayanum) to four inches (Branchiostoma californiense) in length. They live buried in sand in shallow waters along the coasts of warm seas. One species, Amphioxides pelagicus, has been taken at the depth of 1000 fathoms, but whether at the bottom or floating near the surface is not known. The species are very tenacious of life and will endure considerable mutilation. Some of them are found on almost every coast in semi-tropical and tropical regions.
Species of Lancelets.—The Mediterranean species ranges northward to the south of England. Others are found as far north as Chesapeake Bay, San Diego, and Misaki in Japan, where is found a species called Branchiostoma belcheri. The sands at the mouth of San Diego Bay are noted as producing the largest of the species of lancelets, Branchiostoma californiense. From the Bahamas comes the smallest, the type of a distinct genus, Asymmetron lucayanum, distinguished among other things by a projecting tail. Other supposed genera are Amphioxides (pelagicus), dredged in the deep sea off Hawaii and supposed to be pelagic, the mouth without cirri; Epigonichthys (cultellus), from the East Indies, and Heteropleuron (bassanum), from Bass Straits, Australia. These little animals are of great interest to anatomists as giving the clue to the primitive structure of vertebrates. While possibly these have diverged widely from their actual common ancestry with the fishes, they must approach near to these in many ways. Their simplicity is largely primitive, not, as in the Tunicates, the result of subsequent degradation.
Fig. 289.—California Lancelet, Branchiostoma californiense Gill. (From San Diego.)
The lancelets, less than a dozen species in all, constitute a single family, Branchiostomidæ. The principal genus, Branchiostoma, is usually called Amphioxus by anatomists. But while the name Amphioxus, like lancelet, is convenient in vernacular use, it has no standing in systematic nomenclature. The name Branchiostoma was given to lancelets from Naples in 1834, by Costa, while that of Amphioxus, given to specimens from Cornwall, dates from Yarrell's work on the British fishes in 1836. The name Amphioxus may be pleasanter or shorter or more familiar or more correctly descriptive than Branchiostoma, but if so the fact cannot be considered in science as affecting the duty of priority.