672 CHORDATA 



at both ends, with conspicvioiis nuiscle sej^ments (my.otomes) and a 

 dorso-ventral fin at the hinder end. There is no head, the mouth, which 

 is surrounded by long cirri, being ventral in position, skewed a little to 

 the left, and just back of the pointed snout. The anus is near the hinder 

 end, a little to the left of the median line. The anterior portion of the 

 digestive tract forms the extensive pharynx, the walls of which are per- 

 forated by numerous paired gill slits. Through these, respiratory water, 

 which is taken in at the mouth, streams into a peribranchial chamber, 

 wdiieh communicates with the outside through a median ventral pore in 

 the hinder half of the body. In the mid-dorsal as well as the mid- 

 ventral line of the pharynx is a ciliated groove, the latter of which is 

 homologous to the endostyle of tunicates. Back of pharynx the straight 

 intestine passes to the anus, the only digestive gland being the liver, a 

 long diverticulum which extends from the intestine forwards into the 

 peribranchial chamber. The notochord is a cylindrical rod which forms 

 the axial skeleton of the body, through which it extends from tip to tip, 

 lying just above the digestive tube. Above it lies the spinal cord, 

 which possesses a central canal, the anterior end of which expands to 

 fonn a vesicle. Metameric pigment spots, an olfactory pit at the ante- 

 rior end, and bristle-tipped cells in the skin are the organs of special 

 sense. 



The vascular system consists of a ventral longitudinal blood vessel 

 which lies beneath the intestine and pharynx, breaking up into capil- 

 laries (portal system) over the liver, a dorsal vessel over the intestine 

 wliich is double over the pharynx, and lateral vessels connecting these 

 two. The lateral branches in the wall of the pharynx become branchial 

 vessels; in the hinder portion of the animal the lateral vessels break up 

 into capillaries on the wall of the intestine. A heart is not present but 

 the anterior portion of the ventral trunk and others of the larger vessels 

 are contractile; the blood is colorless. The excretory system consists of 

 numerous pairs of nephridia wdiich lie above the upper ends of the gill 

 slits and communicate with the peribranchial chamber. The sexes are 

 separate, the gonads being a series of glands which lie along the side of 

 the body opposite the pharynx, projecting into the peribranchial chamber, 

 into which their products are discharged. 



AmpMoxus was discovered in 1778 by Pallas, who, believing it to be 

 a slug, gave it the name Limax lanceolatus. In 1834 Costa gave the 

 animal the generic name of Branchiostoma. Two years later Yarrell, 

 not knowing of Costa's work, named it Ampliioxus, which has been the 



Quart. Jour. Mic. Sci., Vol. 37, p. 30.3, 1895. '*Cephalochordata," Camb. Nat. Hist, 

 1904. *'Maldive Cephalocbordates," etc., by G. H. Parker, Bull. Mus. Comp. Zool., 

 Vol. 46, p. 39, 1904. 



