752 



COMPARATIVE ANATOMY 



Fig. 437. 



Amphiox ' us lanceola ' tus : 

 a, Anus ; au, eye ; 6, ventral 

 muscles ; c, body cavity ; ch, 

 notochord ; d, intestine ; do 

 and du, dorsal and ventral 

 walls of intestine ; /, fin- 

 seam ; h, skin ; k, gills ; ka, 

 gill-artery ; Ib, liver ; Iv, 

 liver-vein ; m 1 , brain vesicle ; 

 m 2 , spinal marrow ; tng, 

 stomach ; o, mouth ; p, ven- 

 tral pore ; r, dorsal muscle ; 

 s, tail fin ; t, t, aorta ; v, in- 

 testinal vein ; x, boundary 

 between gill intestine and 

 stomach intestine; y, hypo- 

 branchial groove. (After 

 Hackel.) 



ports that serve to funnel the water into the 

 pharynx. The mouth is surrounded by a mem- 

 brane, the velum, which acts as a sphincter 

 muscle. A set of velar tentacles that serve as a 

 grating to strain out the larger particles is de- 

 veloped on the free edges of the velum. 



The pharynx has sometimes upward of fifty 

 or more pairs of gill-clefts (also called branchial 

 apertures) that are separated by partitions in 

 which lie cartilaginous skeletal rods, connected 

 across with one another, forming a sort of bran- 

 chial basket. These apertures serve as means of 

 communication between the pharynx and the 

 atrium (the space between the pharynx and the 

 body-wall). The endostyle (a longitudinal 

 groove on the ventral side of the pharynx), the 

 peripharyngeal and hyperpharyngeal grooves, all 

 secrete mucus in the form of a continuous rope 

 which carries the food along with it to the stom- 

 ach. The atrium is a sort of mantle, composed 

 of folds of the body-wall that enclose the whole 

 branchial apparatus in a voluminous water-filled 

 chamber, the atrial cavity. The atrium is lined 

 with ectoderm and has but one opening to the ex- 

 terior, a posteriorly directed atriopore, which 

 carries off the water that comes through the 

 pharyngeal clefts. The atrium is a protection 

 for the delicate pharynx while the animal is in its 

 sandy burrow and helps to maintain an uninter- 

 rupted current of water. 



ASCIDIANS (TUNICATES) 



In the Ascidians (Fig. 313, IV) the method 

 of food concentration and transportation is simi- 

 lar to that of Amphioxus, although the apparatus 

 which carries on this function seems to be of an 

 improved type more appropriate for a sedentary 

 life. An atrial cavity surrounds the pharynx 

 which in turn is enclosed by a mantle that sur- 

 rounds the whole body. A thick tunic (after 

 which the animal takes its name) covers this 

 mantle. The atriopore is not posterior in direc- 

 tion, but lies close to the mouth and is forwardly 

 directed. The stomach opens near the bottom of 

 the pharynx, and the intestine takes a complete 

 turn and opens forward into the atrium. There is 



