478 MANUAL OF ZOOLOGY. 



The Lancelet is a singular little fish, from one to two inches 

 in length, which is found burrowing in sandbanks, in various 

 seas, but especially in the Mediterranean. The body (fig. 

 259) is semi-transparent, destitute of an exoskeleton, and 

 lanceolate in shape, and is provided with a narrow mem- 

 branous border, of the nature of a median fin, which runs 

 along the whole of the dorsal and part of the 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 ciliated 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 (fig. 259, ), which is 



Fig. 259. The Lancelet (Amphioxus lanceolatiis), enlarged to twi,^, .<, uunm... 010^. 



Mouth ; b Pharyngeal sac ; g Stomach ; h Diverticulum representing the liver ; 



1 Intestine: a Anus : n Notochord : /Rudiments of fin-ravs : 2* / ' ' ' ' 



its natural size. 



IYJ.UULII , i/ j. iicti y ugcdi sen* , OLuniacu , ri J-/IYCI Liuuium icpi eventing the ll> 



Intestine ; a Anus : n Notochord ; y Rudiments of fin-rays ; p Abdominal pore. 



believed to represent the pharynx, and is termed the " pharyn- 

 geal" 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-ciliated mucous mem- 

 brane. 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 ap- 

 pended a long and capacious sac or caecum (h\ which is 

 believed to represent the liver. The intestinal tube termi- 

 nates posteriorly by a distinct anus (a), which is situated at 

 the root of the tail a little to the left of the median line ; and 

 the intestinal mucous membrane is ciliated. Respiration is 

 effected by the admission of water taken in. by the mouth 

 into the branchial sac, having previously passed over the free 



