ORDEKS OF PISHES. 



215 



rounded by a number of cartilaginous filaments. The throat 

 is provided with several leaf-like filaments, which are richly 

 supplied with blood, and are believed to discharge in part the 

 function of gills. The mouth (Fig. 105, m) opens into a 

 dilated chamber, which is believed to represent the pharynx, 

 and is termed the pharyngeal or " branchial " sac. The walls 

 of this chamber {p) are strengthened by numerous cartilagi- 



FiG. 105.^-Diagram of the Lancelet (AmpMoxus lanceolatus). m Mouth with cartilagi- 

 nous filaments ; p Greatly-developed pharynx, or branchial sac, perforated by^ ciliated 

 apertures; i Intestine; a Anus; A Blood-vessels, Tvith pulsating dilatations in place 

 of a heart ; cA Notochord ; n Spinal cord. 



nous filaments, between which are a series of transverse slits 

 or clefts, and the whole is covered with a riohly-cUiated mu- 

 cous membrane. The function of this sac is clearly respiratory, 

 the water from without being admitted through the mouth, 

 passing through the branchial clefts into the abdominal cavity, 

 and finally escaping by means of an aperture placed on the 

 ventral surface a little in front of the anus. From the hinder 

 end of the branchial sac proceeds the alimentary canal, which 

 has appended to it a sac-like organ, believed to represent the 

 liver, and which terminates behind in a distinct anal aperture. 

 There is no heart, and the circulation is entirely eifected by 

 means of several contractile dilatations, developed upon the 

 great blood-vessels (h). The blood itself is colorless. No 

 kidneys have hitherto been discovered, and the reproductive 

 elements are emitted into the abdominal cavity, from which 

 they escape by the pore placed upon the lower surface. 



There is no skeleton properly so called. The notochord 

 (ch) remains throughout life as a semi-gelatinous rod, enclosed 

 in a membranous sheath, and supporting the spinal cord. 

 There is no skuU, and the spinal cord (n) does not expand in 

 front to form a distinct brain. The brain, however, may be 

 said to be represented, as the front portion of the nervous 

 axis gives off nerves to a pair of eyes, and another branch to 



