CHAPTER XXVII 

 THE LEPTOCARDII, OR LANCELETS 



HE Lancelet. The lancelet is a vertebrate reduced to 

 its very lowest terms. The essential organs of ver- 

 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 vertebrae are present. The heart 

 is simply a long contractile tube, hence the name Leptocardii 

 (from Xexros, slender; ttapdia, 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 aro^a, mouth. The body is lanceolate in form, sharp 

 at either end. From this fact arises a third name, Amphioxus, 

 from afjL(f>i, both; ogvs, sharp. Dorsal and anal fins are de- 

 veloped as folds of the skin supported by very slender rays. 



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