GENERAL STUDY OF TYPICAL CHORD ATES n 



Black spots in the neural tube are the pigment cups of simple eyes or light- 

 perceiving organs. These are distributed along the anterior part of the neural 

 tube of Amphioxus and are similar in structure to the eyes of planarians, consist- 

 ing of a pigment cup filled with nerve cells. Experiment has shown that these 

 optic cups are sensitive to light while the eyespot on the anterior end of the 

 brain is insensitive to light. (/) The notochord, an oval mass, larger than the 

 neural tube and directly ventral to it. (g) The atrium, the large cavity occupying 

 the ventral half of the section, (h) The pharynx, the structure occupying the 

 center of the atrium. It is elongated in some regions, heart shaped in others. It 

 consists of separate pieces, the branchial bars, each inclosing in its outer extremity 

 a stiff support, the branchial rod. The spaces between adjacent gill bars are the 

 gill slits or pharyngeal clefts, by means of which the cavity of the pharynx com- 

 municates with the atrium. In the median dorsal line of the pharynx is a deep 

 groove, the epibranchial groove; in the median ventral line, a broad slightly 

 depressed structure, the hypobranchial groove, or endostyle. These grooves 

 secrete mucus in which the minute food particles are caught, (i) The liver, an 

 oval hollow structure present in some sections to the right of the pharynx. It 

 is composed of tall epithelial cells, which probably produce a digestive secretion. 

 (J) The gonads. In cross-sections of larger specimens these are found as 

 masses, one on each side of and projecting into the atrium. Draw the cross- 

 section. 



For further details of the anatomy of Amphioxus consult P and H, pages 

 44-56; CNH, Vol. VII, pages 112-30; N, pages 30-42. 



B. A TUNICATE 



1. External anatomy. — Obtain a specimen and place in a dish of water. 

 It is an oval saclike creature, scarcely recognizable as an animal. The animal 

 is in life permanently attached to rocks or other objects. The end which was 

 attached is recognizable by its rough and irregular form and by the fragments 

 of wood or other materials which adhere to it. The opposite end, which in the 

 living state extends free into the water, bears two openings, the siphons. When 

 the animal is gently squeezed, jets of water are observed to squirt from the 

 siphons; hence the name sea squirt popularly applied to these animals. The 

 upper of the two siphons is the oral or incurrent siphon and is the degenerate 

 anterior end of the animal; the lower siphon is the atrial or excurrent siphon 

 and represents the posterior end. The anteroposterior axis of the tunicates is 

 bent into a U-shape. The region of the body between the two siphons is there- 

 fore dorsal, and the remaining and much larger part of the surface is ventral. 

 When the animal is undisturbed, the siphons are widely expanded, and a current 

 of water passes into the oral siphon and out of the atrial siphon. When disturbed 

 and also generally in the preserved state, the siphons are closed and retracted. 

 Closure and reaction is accomplished by circular and longitudinal muscles located 



