COMPARATIVE ANATOMY 3 



forated by numerous gill slits. In respiration water passes 

 through the gill slits into a large cavity, the atrium, which con- 

 nects with the atriopore. The skeletal structures supporting 

 the gills are the bra?ichial bars. The liver is a simple unbranched 

 diverticulum on the ventral side of the intestine. Dorsal to 

 the intestine lies the notochord, and just above this, the nerve 

 cord. 



Study cross sections of amphioxus cut through different 

 regions of the body. The outer covering, the epidermis, is 

 composed of a single layer of cells. The dermis, beneath the 

 epidermis, is a thin layer of connective tissue. Identify the 

 myomeres, the thick masses of muscle; notochord; nerve cord; 

 dorsal fin ray, a connective tissue support for the dorsal fin; 

 pharynx with its gill slits and branchial bars in sections of the 

 anterior end; atrium; intestine and liver, in sections behind the 

 pharynx; metapleural folds. The ciliated groove on the ventral 

 side of the pharynx is the endostyle, a structure often considered 

 as the evolutionary forerunner of the thyroid gland. 



Lamprey — Class Cyclostomata 



Examine whole and dissected lampreys. There are two 

 dorsal fins and a caudal fin, but paired appendages are absent. 

 The skin is soft and slimy and contains numerous glands. True 

 jaws are missing. The large cavity on the ventral side of the 

 head, lined with horny teeth, is the buccal funnel. The tongue, 

 also covered with horny teeth, lies at the bottom of the funnel. 

 There is but one nostril, on the dorsal side of the head. The eyes 

 are indistinct. Posterior to the eye, on each side, is a row of 

 seven gill slits. The anus opens on the ventral side near the 

 caudal fin. Close behind the anus is the urogenital aperature. 



In a median sagittal section locate the tongue and its large 

 retractor muscles; the pharynx and gill slits; the muscular heart; 

 the large notochord and above it the small brain and spinal cord; 

 the thin ribbon-like intestine; the liver, close behind the heart; 

 and further back, the kidneys and gonads suspended from the 

 dorsal body wall. 



Identify these same structures in cross sections of lampreys 

 cut at various levels. Under the notochord are three blood 

 vessels, a median aorta and two lateral cardinal veins. 



