416 THE AMERICAN BIONTHLY [Detv 



8. PrincipalPartsoftheTrunk AND Post- Abdomen. 

 — Spl't tliesrnelt in twoiii a vertical plane, pa^^siii^ a liille 

 to one side of the middle line. Cut across the ribs of one 

 side and remove the lesser half entirely from the body. 

 Notice that, the y)ost-abdomen is made up entirely of 

 large masses of bluish grey viuscular tissue, and that the- 

 same is true of the dorsal j»ortion of the trunk region; 

 but that in tlie trunk below the back-bone the muscular 

 tissue is [)ushed aside, forming a thin layer in the wall 

 of the 6oc?2/-ca?)iY2/ (coeZom), and that the space thus gained 

 is occupied by the viscera (organs of digestion, etc. The 

 viscera will be studied later). 



Locate the back- bone spinal-column, extending th& 

 lenjfth of tiie body from the l)i-ain case in front to the base 

 of the caudal fin. Examine it and recognize the situa- 

 tion of the successive pieces, centra, of which it is made; 

 each centrum bears on the dorsal side a neural spine 

 which runs U[)ward and backward, the course of the neural 

 spine is often indicated by bb^od vessels which run beside 

 it. In the po^t-abdominal region a similar series of 

 spines run ventrally, these are called haemal spines be- 

 cause the dorsal aorta is related to thein. In the trunk 

 region there are no haemal spines ; but the ribs on each 

 side in pairs articulate with the centra; the, ril>s pass in- 

 sensibly into haemal spines. Follow the body-cavity for- 

 wards and note that it runs between and beneath the 

 gilK--, where you will see the deep red heart. It extentU 

 posteriorly to the cloaca. It is everywhere lined with 

 a delicate silvery lining layer, the peritoneum. Dissect 

 away some of the peritoneum and see that the wall is 

 composed of muscular tissue, and that this is composed 

 of short fibres which run from one rib to the next (seen 

 better in a specimen which has been boiled, vid par. 12). 

 Note that tht^re is no breast bone for articulation with 

 the ribs at their distal extremitits; also that the body 

 cavity is not divided by a diaphragm into thorax and 



