I I 4 CEPHALOCHORDATA 



connected with it a simple hepatic portal blood system. There 

 is a respiratory circulation, the contractile ventral vessel which 

 represents the heart sending the colourless blood forward to the 

 respiratory pharynx to be purified. The body-wall is segmented 

 into over fifty myotomes. There are numerous separate nephridia 

 which develop from the mesoderm and open into the atrium. 

 The brain remains undeveloped, being scarcely distinct from the 

 spinal cord. There are two pairs of cerebral nerves, .and many 

 spinal, in which the dorsal and ventral roots or nerves do not 

 unite. The sense-organs are simple ; there are no paired eyes 

 and no auditory organs. The sexes are separate ; the gonads are 

 metamerically arranged on the body-wall, and have no ducts : 

 they burst into the atrium. In the development the segmenta- 

 tion is complete, a gastrula is formed by invagination, the 

 nervous system is formed from the dorsal epiblast, the noto- 

 chord from the hypoblast, and the mesoderm arises from meta- 

 meric coelomic pouches. The body -cavity is an enterocoele. 

 The gill-slits are at first perforations of the body-wall opening 

 from the pharynx to the exterior, which later become enclosed 

 by the development of the atrium. 



Anatomy. 



External Characters. — Amphioxus^ is about 1-^ to 2i 

 inches in length, slender, somewhat translucent, and pointed at 

 both ends (Fig. 69). It lives in shallow water and burrows in 

 the sand, head first, with great rapidity. It frequently remains 

 with the anterior end protruding from the sand. When on the 

 surface it lies on one side. It is said to swim freely at night. 

 The head end is rather the thicker, and the anterior two-thirds 

 of the ventral surface are flattened (Fig. 70, A), and may be 

 slightly ridged longitudinally. The lateral edges of this flat 

 area project as metapleural folds (Fig. 70, mt.pl), which begin 

 anteriorly at the edges of the external mouth, and die away in 

 the middle line posteriorly behind a median opening, the 

 atriopore (Fig. 70, atrjj). From this point a ventral median 

 fin (vent.f) extends backwards around the pointed posterior end 



' Although the correct systematic name of the commonest species is Branehio- 

 stoma lanceolat'um (Pallas), it is convenient in non- systematic usage to employ 

 the term " Amphioxus,'' which is in general use in zoological laboratories. 



