10 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



the exterior through the atriopore. Amphioxus maintains a continuous circula- 

 tion of water through its digestive tract; water enters the mouth, passes through 

 the walls of the pharynx into the atrium, and out of the atriopore. Make a 

 drawing of the animal from the side. 



2. Internal anatomy. The internal anatomy is most easily studied on small 

 mounted specimens. Examine with the low power of the microscope. Identify: 

 the various parts of the fin, containing rectangular bodies, the fin rays, serving 

 as the skeletal support of the fins; the muscle segments; and the digestive tract, 

 occupying the ventral half of the body. The parts of the digestive tract may 

 be studied in some detail. Note again the oral hood with its cirri. The vesti- 

 bule is narrowed posteriorly by a membrane, the -velum, pierced by a small open- 

 ing, the mouth. The mouth opens into a wide cavity, the pharynx, extending 

 half the length of the body. Its walls are perforated by numerous oblique 

 slits, the gill slits or pharyngeal clefts. The solid portions of the pharyngeal wall 

 between the gill clefts are called the branchial bars and each is supported by an 

 internal skeletal branchial rod. Surrounding the pharynx is a large cavity, the 

 atrium, the ventral boundary of which is visible as a line below the pharynx. 

 This line may be traced to the atriopore. The posterior end of the pharynx 

 opens into a tubular intestine which extends straight to the anus. The first part 

 of this intestine for a short distance posterior to the pharynx is narrow and dor- 

 sally located ; the intestine then widens suddenly and from this widened part 

 a blind sac, the liver, extends forward beneath the narrowed portion. 



Immediately dorsal to the digestive tract and nearly as wide as the pharynx 

 is a rod, the notochord, extending the length of the body. It will be seen by 

 focusing down through the myotomes and is most distinct in the head, where it 

 runs forward nearly to the extreme tip. Just above the notochord is situated 

 the much smaller neural tube, best recognized by the row of black pigment spots 

 which it bears. These pigment spots have been shown to be sensitive to light. 

 Draw the whole mount, showing its structure. 



3. Cross-section through the pharyngeal region. Examine the cross- 

 section with the low power and identify the following: (a) The epidermis, the 

 outer covering of the body composed of a single layer of columnar epithelial cells. 

 (b] The dorsal median projection, the dorsal fin, containing an oval mass, the 

 fin ray, which supports it. (c) The two ventrolateral projections, the meta- 

 pleural folds. There are a number of smaller folds in the ventral wall between 

 the two metapleural folds, (d) The myotomes, a series of circular masses filling 

 the dorsal and lateral portions of the body wall, and separated from each other 

 by connective tissue partitions. The myotomes are thick dorsally and thin out 

 ventrally. Transverse muscles are present in the ventral body wall, just above 

 the small folds of the epidermis, (e) The neural tube, a median dorsal mass, 

 oval or trapezoidal in section, lying between the dorsal portions of the myotomes, 

 below the fin ray. Observe that it contains a central canal, the neurocoel. 



