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notochord ; laterally and ventrally it (and also a portion 

 of the intestine) is surrounded by the atrium, an extensive 

 space between gut and body wall. The side walls of the 

 pharynx have numerous gill-slits directed downwards and 

 slightly backwards. The narrow portions between the 

 slits are supported by double chitinous rods with forked 

 or split lower ends (primary bars) ; and each slit is double, 

 being divided, during later development, by a secondary 

 downgrowth, which is stiffened by a single unsplit rod 

 (secondary or tongue-bar) containing a coelomic cavity. 

 There are also short horizontal bars (synapticulae) con- 

 necting the primary and tongue-bars. 



This gill-slit or branchial region of the pharynx is respira- 

 tory ; it is- richly ciliated and has numerous blood-vessels. 

 There the blood is aerated by the water. Passing through 

 the gill-slits into the atrium, the water then passes out at 

 the atriopore. 



Describe the Skeletal Structure of Amphioxus. 



Between the dorsal spinal cord and the alimentary canal 

 there is a median flexible supporting rod, the notochord, 

 which extends throughout the body from end to end ; 

 it is composed of large vacuolated turgid cells, and it has a 

 double sheath of connective tissue, the outer one being 

 part of the connective-tissue framework of the body. 



The margin of the oral hood has an oval buccal ring ; 

 it consists of a number of pieces, and each bears an offshoot 

 which supports a buccal cirrus. 



The chitinous primary and tongue-bars and synapticulae 

 of the gill-slit region of the pharynx, and the fin-rays, 

 are also structures of a skeletal nature. 



Write an Account of (I) the Nervous System, and (2) the 

 Circulatory System of Amphioxus. 



1. The central nervous system is a dorsal spinal cord, 

 nearly as long as the notochord ^r\ pitna.^^ a.hnve it. 

 The spinal cord is enclosed in a sheathof connective tissue ; 

 and it has a narrow canal which expands into a cerebral 

 vesicle (ventricle of " brain ") at the front end. From the 

 anterior wall of the vesicle there is a small hollow out- 

 growth, which is connected with an ectodermal olfactory 

 pit. There is also a pigment spot (" eye ") in the vesicle 

 wall at the extreme front end of the cord ; and along the 

 canal, at regular intervals, there are other pigment spots, 

 probably sensitive to light. 



