PHARYNGOBRANCHII. 



401 



ventral surface, and expands at the tail to form a lancet-shaped 

 caudal fin. No true paired fins, representing the anterior and 

 posterior limbs, are present. The mouth is a longitudinal 

 fissure, situated at the front of the head, and destitute of jaws. 

 It is surrounded by a cartilaginous ring, composed of many 

 pieces, which give off prolongations, so as to form a number 

 of cartilaginous filaments or ''cirri" on each side of 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 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 



Fig. 169 Diagram of the Lancelet (Amphioxus). m Mouth, surrounded by cartila- 

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

 terminating in anus (11); h, Haemal system, with pulsating dilatations ; ch Notochord ; 

 n Spinal cord. 



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 



2 C 



