396 COLLEGE ZOOLOGY 



Internal Anatomy and Physiology. — Skeleton. — Am- 

 phioxus has a well-developed axial support, the notochord (Figs. 

 341-342, nch), lying near the dorsal surface and extending almost 

 the entire length of the body. The notochord is composed of 

 vacuolated cells which are made turgid by their fluid contents 

 and are, therefore, resistant. Other skeletal structures are the 

 connective tissue rods which form the fin-rays (Fig. 341, //-.), and 

 similar structures (Fig. 343, sk) that support the cirri (cir) of 

 the oral hood (or.fhd). 



Digestive System (Fig. 343). — The food of Amphioxus con- 

 sists of minute organisms which are carried into the mouth with 

 the current of water produced by cilia on the gills (compare with 

 mussel, p. 246). The mouth {mill) is an opening in a membrane, 

 the velum (vl), and may be closed by circular muscle fibers which 

 surround it. Twelve sensorv-oral or velar tentacles (vl.t) pro- 

 tect the mouth, and, when folded across it, act as a strainer, thus 

 preventing the entrance of coarse, solid objects. The funnel- 

 shaped vestibule is the cavity of the oral hood (or.fhd). The 

 twenty-two ciliated cirri (cir) which project from the edge of 

 the oral hood are provided with sensory cells. The inner wall of 

 the oral hood bears a number of ciliated lobes and is known as 

 the wheel organ because its cilia appear to produce a rotatory 

 movement. Water is forced into the mouth by the cilia. 



The mouth opens into a large, laterally compressed pharynx 

 (Fig. 343, ph; Fig. 342). A ciliated dorsal indentation in the 

 pharynx is called the hyperbranchial groove (Fig. 342, hy.). A 

 ventral groove, the endostyle (en), is also present. The endo- 

 style consists of a median ciliated region with a glandular portion 

 on either side. The glands secrete strings of mucus (compare 

 tunicate, p. 391) in which food particles are entangled. The 

 cilia then drive this mucus forward by way of two peri- 

 pharyngeal grooves into the hyperbranchial groove. From here 

 it is carried by the hyperpharyngeal cilia into the intestine (Fig. 

 343, int). A ventral finger-shaped diverticulum of the intestine 

 is known as the liver (Ir), or hepatic c cecum, since it is supposed 



