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 ~leptos~, slender; ~kardia~, 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 ~stoma~, mouth. The body is lanceolate
in form, sharp at either end. From this fact arises a third name,
_Amphioxus_, from ~amphi~, both; ~oxys~, 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.
[Illustration: 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.
The name _Acraniata_ (without skull) is often used for the lower
Chordates taken collectively, and it is sometimes applied to the
lancelets alone. It refers to those chordate forms which have no
skull nor brain, as distinguished from the _Craniota_, or forms with
a distinct brain having a bony or cartilaginous capsule for its
protection.
=Origin of Lancelets.=--It is doubtless true, as Dr. Willey suggests,
that the Vertebrates became separated from their worm-like ancestry
through "the concentration of the central nervous system along the
dorsal side of the body and its conversion into a hollow tube."
Besides this trait two others are common to all of them, the presence
of the gill-slits and that of the notochord. The gill-slits may have
served primarily to relieve the stomach of water, as in the lowest
forms they enter directly into the body-cavity. The primitive function
of the notochord is still far from clear, but its ultimate use of its
structures in affording protection and in furnishing a fulcrum for the
muscles and limbs is of the greatest importance in the processes of
life.
[Illustration: FIG. 289_a_.--Gill-basket of Lamprey.]