Class 1. Leptoccu'dii. 



355 



posteriorly. Above this lies the central nervous systenija 

 long cord-like organ without specialised brain; its central canal 

 communicates with the exterior in front. There is an unpaired 

 eye in the form of a pigmented spot in the anterior part of the 



Fig. 293. Diagrammatic longitudinal section of Amphioxus. a anus, c notoohord, 

 gill sac, m stomach, n central nervous system, p atrium, p' atriopore. — Orig. 



nervous system ; paired eyes and auditory organs are absent. The 

 musculature is arranged as in Fish, the muscle-fibres transversely 

 striated. Below the notochord lies the alimentary canal, 

 beginning at the anterior end of the animal with a mouth, surrounded 

 by a number of projecting tentacles (cirrhi), this leads into a large 

 pharynx, perforated by numerous transverse slits, and extending 

 back through a large region 'of the body. Behind, the pharynx leads 

 into the stomach, which is furnished with a large evagination, the 

 liver; the intestine is short and straight and opens ventrally, not far 

 from the hind end, so that the length of the tail is not great. The gill- 

 clefts do not lead direct to the exterior, but open into an atrium 

 surrounding the pharynx, and this again opens 

 ventrally, anterior to the anus. In quite young- 

 animals the branchial clefts open on the surface, but 

 later a longitudinal fold develops above them on 

 each side, and eventually grows round them, the 

 two uniting on the ventral surface. The vascular 

 system is remarkable for the absence of a 

 specialised heart, but all the vessels are 

 pulsatile. Below the pharynx is an unpaired vessel, 

 which receives the venous blood from the body, and 

 sends branches to the gill bars ; from the latter the 

 blood is collected into an aorta running below the 

 notochord. A hepatic portal system is present as in 

 other Vertebrata. Eed blood corpuscles are wanting. 

 Numerous short ciliated tubes, arranged in a row on 

 either side, serve for excretory organs; each 

 tube has a single dorsal opening into the atrium, 

 several into the body-cavity. The sexes are separate, 

 •organs are represented by several pairs of ovaries or testes, which 



A A 2 



Fig. 294. 

 Transverse sec- 

 tion tlirough the 

 anterior region 

 of the body of 

 Amphioxus ; dia- 

 grammatic, n 

 spinal cord, ch 

 notochord, g pha- 

 rynx, p atrium, 

 k gonad. — Orig. 



The sexual 



