neural tube 



Figure 1-1. Amphioxus swimming (when disturbed) and feeding. 

 (After Newman) 



animal, beyond the neural tube anteriorly. Above the neural 

 tube is a series of dorsal rays supporting the dorsal fin fold. 

 The body wall is smooth and covered by a cuticle. Within 

 the anterior third of the animal, the slits of the pharynx are 

 observed. These open into an atrial cavity (Figure 1-4), 

 which exits to the outside by an atrial pore (Figure 1-5) 

 about two-thirds of the way back on the body (Figure 1-4). 

 The atrium is enclosed below by a sheet of tissue in which 

 there are transverse muscle fibers. The body wall e.\tends 

 down beyond this plate as metapleural folds. The atrial 

 cavity extends posteriorly beyond the atriopore as bilateral 

 sacs, the one on the right side being the larger. There are 



Hatschel('s fossa in rotatory organ 



Hatschei<'s nephridium 

 arterial expansion 



Figure 1-3. Cross section tlirough oral funnel showing Hotscfiek's 

 fossa in the rotatory organ. 



also anterior digitations into the restricted dorsolateral 

 coelomic spaces above the pharynx. 



The <-shaped muscle segments (myomeres), whose 

 points are directed anteriorly, are also visible through the 

 cover of the body. There are about 60 pairs of muscle seg- 

 ments in the body and 180 slits in the phar>'nx. The differ- 

 ence in number is due to the fact that the slits continue to 

 multiply in the course of development after the number of 

 muscle segments has become fixed. 



The digestive tract begins at the posterior end of the 

 pharynx, which tapers to an esophagus (Figure 1-5). A short 



ciliated fossa (Kdllicl<er's pit) 



pigment area 



rostral cavities 



Hatschel<'s fossa (dashed line) 

 Hotschek's 



myotome 



fin ray in sclerocoel 



oral hood 



velum and tentacles 

 Figure 1-2. Anterior end of Amphioxus. (After Franz, 1927) 



CHORDATES, PROTOCHORDATES, AND THEIR RELATIONSHIPS 



