108 LAHOHATORY STUDIES IN ZnOJjOGY 



as an axial skolcloii. Other skeletal parts are found supporlinj; 

 the oral cirri, gill slits, and mesal fin fold. 



I* The Central Nervous System. — The brain and spinal cord lie 

 immediately above the notochord. Determine the extent and 

 relations of the central nervous organs. 



Upon the plate of the left lateral aspect of Branchiostoma add 

 features of external and internal structure which have been 

 observed in the above study. 



Study of Transverse Sections. — The relations of internal 

 organs are more readily understood through a study of transverse 

 sections. For this purpose prepared slides through the region 

 of the pharynx and liver are most instructive. 



Use a low-power microscope to determine the shape and general 

 location and relations of internal structures. A higher magnifica- 

 tion may be used for the study of details. An explanation of the 

 stains used in preparing the slides will be given by the instructors. 



The section chosen for description here is one passing through 

 the body at the level of the liver. In section the body is tri- 

 angular in shape. In the midvertical portion may be seen the 

 compressed pharynx on the right of which is the liver. The 

 gonads on either side appear as elevations from the walls of 

 the body encroaching upon the atrial cavity. 



The epidermis consists of a single layer of cells supported by an 

 underlying thin layer of connective tissue, the dermis. 



Identify and locate pharyn.x, notochord, and spinal cord. 

 Determine the following: (1) the nature of the notochordal cells, 

 notochordal sheath, and the gill slits; (2) the nature of the spinal 

 cord ; (3) the nature and relations of muscle segments (myomeres) ; 

 (4) the nature and relations of muscle partitions (myotomes) ; (5) 

 the dorsal groove of the pharynx {epipharyngeal groove) and the 

 ventral groove (endostyle). These grooves serve to entangle the 

 minute particles of food and transport them toward the intestine. 



The coelom is much reduced, having been encroached upon 

 by the extensive atrial cavity. The remnants of the coelom 

 consist of paired cavities at the sides of the epipharyngeal groove, 

 a single space ventrad of the endostyle, a space between the walls 

 of the liver and the atrial epithelium, and the space surrounding 

 the gonads. 



The larger blood vessels can often be recognized by the 

 coagulated blood which they contain. The dorsal aortce comprise 

 a pair of vessels lying one on each side of the epibranchial groove. 



