CHAPTER XII 
THE LEPTOCARDII, OR LANCELETS 
AIHE Lancelet.—The lancelet is a vertebrate reduced to 
mice. | ER : 
4 [ i its very lowest terms. The essential organs of ver- 
ms. A 2 
tebrate life are there, but each one in its simplest form 
unspecialized and with structure and function feebly differen- 
tiated. The skeleton consists of a cartilaginous notochord in- 
closed in a membranous sheath. There is no skull. No limbs, 
no conspicuous processes, and no vertebre are present. The heart 
is simply a long contractile tube, hence the name Leptocardit 
(from Aewzos, slender; «apdia, 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 respira- 
tion 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 seg- 
mentally 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 orova, mouth. The body is lanceolate in form, sharp 
at either end. From this fact arises a third name, Amphioxus, 
from api, both; ogvs, sharp. Dorsal and anal fins are de- 
veloped as folds of the skin supported by very slender rays. 
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