338 MANUAL OF ELEMENTARY ZOOLOGY 



epithelium, and around it lie nerve cells, but there are 

 no dorsal and ventral horns. The outer part of the cord 

 is composed of non-medullated fibres. At the anterior 

 end the canal widens out into a cerebral vesicle, which in 

 the larva communicates by a pore with a ciliated funnel 

 known as the olfactory pit, on the dorsal surface of the 

 left side of the body. In the adult this opening is lost, 

 though the pit remains. The first two pairs of nerves are 

 specialised as cerebral nerves. The first pair arise from the 

 lower side of the anterior end of the cord, the second pair 

 from the dorsal surface behind the cerebral vesicle. These 

 pairs are symmetrical. They are distributed to the epidermis 

 of the snout and are sensory in function. The remaining 

 nerves are not symmetrical, but alternate with one another 

 on the two sides, in correlation with the alternation of the 

 myomeres. Each corresponds to a dorsal or a ventral root 

 of a spinal nerve of the frog, the ventral roots being placed 

 in front of the dorsal. The roots do not join, the ventral, 

 which are groups of slender rootlets, passing direct to the 

 muscles, and the dorsal, which are compact, but have no 

 ganglia, passing in the septa between the myomeres to 

 the epidermis. The sense organs are few and simple. 

 Supposed tactile cells bearing short, stiff processes are 

 scattered among the ordinary ectoderm cells, especially 

 at the front end of the body and around the mouth. 

 The ciliated pit already described is supposed to be 

 olfactory. A mass of pigment which lies in the front 

 wall of the cerebral vesicle is not sensitive to light, 

 but small groups of pigmented organs which occur at 

 intervals on the lower side of the canal in the cord 

 appear to be so. 



The sexes of the lancelet are separate, but show no 

 differences save in the nature of the gonads. 

 organs!"" 1 ™ These are cubical bodies, twenty-six on each 

 side, placed in the wall of the atrium, into 

 which they shed their germs by rupture of their walls. 

 Each corresponds to one of the myomeres and consists of 

 a closed sac, on whose wall the germs arise. The egg is 

 minute, but contains yolk granules. Fertilisation takes 

 place in the water. 



The lancelet is an example of a group of animals known 



