354 



MANUAL OF ZOOLOGY. 



the mouth. (Hence the name of Cirrostomi, proposed by 

 Professor Owen for the order.) The throat is provided on 

 each side with vascular lamellae, which are believed by Owen 



Fig. 132. Diagram of the Lancelet (Atnphioxus), m Mouth, surrounded by cartila- 

 ginous cirri; p Greatly dilated pharynx, perforated by ciliated clefts; i Intestine, 

 terminating in anus (a); h Haemal system, with pulsating dilatations ; ch Notochord ; 

 n Spinal cord. 



to perform the function of free branchial filaments. The mouth 

 leads into a dilated chamber, which is believed to represent the 

 pharynx, and is termed the " pharyngeal " or " branchial sac." 

 It is an elongated chamber, the walls of which are strengthened 

 by numerous cartilaginous filaments, between which is a series 

 of transverse slits or clefts, the whole covered by a richly cili- 

 ated mucous membrane. This branchial dilatation has given 

 rise to the name Branchiostoma, often applied to the Lancelet. 

 Posteriorly the branchial sac opens into an alimentary canal, to 

 which is appended a long and capacious sac or caecum, which 

 is believed to represent the liver. The intestinal tube termi- 

 nates posteriorly by a distinct anus. Respiration is effected 

 by the admission of water taken in by the mouth into the 

 branchial sac, having previously passed over the free branchial 

 filaments before mentioned. The water passes through the slits 

 in the branchial sac, and thus gains access to the abdominal 

 cavity, from which it escapes by means of an aperture with 

 contractile margins situated a little in front of the anus, and 

 called the " abdominal pore." There is no distinct heart, and 

 the circulation is entirely effected by means of several rhythmi- 

 cally contractile dilatations which are developed upon several 

 of the great blood-vessels. The blood itself is colourless. No 

 kidneys have as yet been discovered, and there is no lymphatic 

 system. There is no skeleton properly so called. In place of 

 the vertebral column, and constituting the whole endoskeleton, 

 is the semi-gelatinous cellular notochord, enclosed in a fibrous 

 sheath, and giving off fibrous arches above and below. The 

 notochord is, further, peculiar in this, that it is prolonged quite 

 to the anterior end of the body, whereas in all other Vertebrates 



