440 



CIIORDATA 



to a rudimentary cerebral vesicle, with a pi,i:;ment spot, the nulimentary 

 eve (ail), but other ])lace# in the spinal cord are sensitive to light. The 

 neural canal long renrains opiir in froirl, the opening (iii-iiroporc) being at 

 the bollom of a pit, once regarded as an olfactory organ. 



The alimentary canal begins with the o\'al mout.h (<>), surrounded by 

 cirri; then comes a fold (I'cliiiii), followed by the pharynx, perforated by 

 numerous gill clefts, which extend o\ er a thirtl of the alimentary canal. 

 Between the clefts are elastic gill arches (lig. 404, kl') to support the walls. 

 In the young the clefts open directly to the exterior; then a fold grows 



Fir,. 404. — Section of the gill region of .1 /»/)/; /iiv»\ (after T.iinke.^ter ami rtoveri). 

 J, aorta ileseL-mlens; /', ]icriljranchial space; (", notoelioril ; ^0, ealoin l^braneliial bo*-\v 

 ^-avity); i\ hypolirancliial groove, bcneatli it tlie aorta aseen.iens; c. gonad; kh, giil 

 nrclie-^; ktl, pharynx; /, li\-er; ni , museles; ?;, nephrjelia, on the left witli an arrow; r, 

 ipinal cord; sii, spinal nerve; 5/), gill slit. 



dowir on either side, the two folds unititig below to enclose a pcrihraiicJiia! 

 chamber (l>) from which the water escapes b)- an opening, alriopore (40;,,). 

 behind the middle of the body. On the lloor of the pharynx is the hvpo- 

 hranchial groove, a ciliated tract which conducts food to the digesli\c part 

 of the canal. Thiseinlsat the anus (j), on ihe left side of the bodv and h;is 

 a liver connected with it in front, extending into the peribranchial chamber 

 (figs- 493> 494, h). The vascular system, with colorless blood, consists of 

 a dorsal arterial (a) and a ventral \enous trunk coiinected by lateral loops 



