STRUCTURE OF THE LANCELET. 419 



the single organ of smell. The organ of hearing is entirely 

 wantino'. This defective evolution of the hioher sense- 

 organs is probably in great measure explicable as not 

 original, but as a degeneration. 



Below the notochord runs a very simple intestinal canal, 

 a tube, which, on the ventral side of the little animal, opens 

 in front in a mouth, and at the back in an anus. The mouth 

 is oval, and surrounded by a cartilaginous circle, on which 

 are 20 to 30 filaments of cartilage (organs of taste) (Fig. 

 151, a). By a contraction in the centre, the intestinal canal 

 divides in the centre into two very different parts, of about 

 equal length. The anterior division acts as a respiratory 

 organ, the posterior end as a digestive organ. The anterior 

 half forms a wide gill-body, the lattice-like wall of which 

 is pierced by numerous gill-openings (Fig. 151, d, and Plate 

 XI. Fig. 15, k). The delicate bars of the gill-body, between the 

 openings, are supported by small, firm parallel staves, which 

 are connected together in pairs by cross-staves. The water 

 which the Amphioxus takes in through its mouth passes 

 through these openings in the gill-body into the large gill- 

 cavity which surround the gill-body, and then passes further 

 back and out through the breath-hole, or gill-pore (j^orus 

 hranchialis ; Fig. 151, c). On the ventral side of the gill- 

 body there is, along the central line, a ciliated groove 

 (the hypobranchial groove), which also occurs in Ascidians 

 and in the larvie of Cyclostomi ; it is of interest because 

 from it in the higher Vertebrates is developed the thyroid 

 cartilage on the throat (on the lower part of the so-called 

 Adam's apple; Fig. 15, y). 



Behind the breathing, or respiratory part of the intestinal 



canal comes, secondly, the digestive part. The small bodies 



20 



