38 VERTEBRATE ZOOLOGY 



testine. There its load of food-particles is digested and the mucus 

 itself passes out of the anus. 



It is of considerable interest to note in this connection that this 

 complex food-concentrating mechanism is found in the urochordates 

 and in the Ammocoetes larva of the lamprey eels, but nowhere else in 

 the animal kingdom. Thus it furnishes a point of connection be- 

 tween Amphioxus and the lower chordates, on the one hand, and be- 

 tween Amphioxus and the vertebrates, on the other. It should also 

 be noted that the primary function of the pharyngeal apparatus, in- 

 cluding pharyngeal clefts, appears to be that of food concentration 

 rather than that of respiration. 



Characters of Amphioxus 



1. Characters Associated with the Reduced Head. — ^The brain 

 is extremely small, hardly as large in diameter as the rest of the neural 

 tube, (Figs. 11, A and B). There are but two pairs of cranial nerves 

 which have been called olfactory and optic; but in so reduced a brain 

 homologies are uncertain. The sense organs consist of a median ol- 

 factory funnel opening into the neurocoel, a median rudimentary eye- 

 spot on the anterior end of the brain, representing probably the last 

 rudiments of the ancestral paired ej'es. The notochord extends the 

 entire length of the body, projecting in front of the brain. This may 

 mean that the brain has retreated from a primitive anterior position. 

 There is no cranium. Possibly the ancestral chordate had some sort 

 of cranium. 



2. Characters that make up the Food-Concentrating Mechanism. — 

 Since the method of feeding has been described these characters need 

 only be Usted. The mouth is an oral funnel bounded by ciliated 

 buccal tentacles with cartilaginous supports that serve to funnel the 

 water into the pharynx. Separating the oral funnel from the pharynx 

 is a velum composed of a membrane with sphincter muscles and a set 

 of velar tentacles that serve as a grating and strain out the larger par- 

 ticles. The pharnyx has sometimes upwards of fifty pairs of clefts 

 that are separated by partitions in which lie cartilaginous skeletal 

 rods connected across with one another, forming a sort of branchial 

 basket. The endostyle, peripharyngeal, and hyper pharyngeal grooves 

 secrete mucus and propel the food to the stomach by means of the 

 mucous rope food carrier. The atrium is a sort of mantle composed of 

 folds of the body wall that incloses the whole branchial apparatus in 



