THE ANCESTRY OF THE VERTEBRATES 531 



ber and disposed in lateral rows, but which usually become al- 

 most indefinitely multiplied, and in many cases transform the 

 entire pharyngeal wall into a structure resembling basket-work. 

 Thus the cloacal chamber, except for the relation of the anal 

 outlet, is precisely similar to the peribranchial chamber of 

 Amphioxus, the excurrent orifice being the equivalent of the 

 atriopore. The pharynx in the two animals is also similar in 

 the presence and secondary multiplication of its gill-slits, and 

 here also there is an endostyle, which lies along that side of the 

 pharynx which in the free-swimming larva is ventral. The 

 nervous system consists of a single ganglion, placed in the adult 

 between the two arms of the U-shaped intestine, but dorsal 

 to it in the larva, and the vascular system is represented by 

 a ventrally placed heart. The reproductive gonads lie in the 

 bend of the intestine, and open by ducts of their own into the 

 cloacal chamber. 



These numerous suggestions of an organization akin to that 

 of Amphioxus which are noticeable in the adult are far more 

 apparent in the larva (Fig. 146, a). In this stage the ani- 

 mal is tadpole-like and possesses a long tail, flattened lat- 

 erally and provided with a typical notochord, similar in its de- 

 velopment to that of Amphioxus. A series of segmentally 

 arranged muscles, separated by myocommata, is also found 

 here, especially developed posteriorly. The nervous system 

 appears in the form of a prolonged neural tube and expands at 

 its anterior end into a sensory vesicle (brain) which is pro- 

 vided with a pigment speck that forms a primitive eye, and a 

 somewhat problematic organ usually interpreted as an ear. 

 although both organs are exceedingly simple in construction. 

 In this larval condition the gill-slits are few in number and 

 simple in arrangement, and the endostyle is in the proper po- 

 sition for comparison with that of Amphioxus. The heart 

 also lies ventrally and just posterior to the oesophagus. 



The changes that take place during the assumption of the 

 sessile position are shown in Fig. 146, b and c, and are ex- 

 plicable by assuming a twist or rotation of the animal towards 

 the left after fixation, by means of two papillae of attachment. 



