XIII 



PHYLUM CHORDATA 



49 



origin in front of the first rnyomere, the first from the anterior 

 extremity of the brain, the second from its dorsal region : they 

 are both distributed to the snout, their branches being pro- 

 vided towards their extremities with numerous ganglia containing 

 nerve-cells. The spinal nerves are segmentally arranged, and, in 

 correspondence with the disposition of the myomeres, those of the 

 right and left sides arise alternately, and not opposite one another 

 (Fig. 703). In each segment there are two nerves on each side, a 

 dorsal nerve, arising by a single root from the 

 dorsal aspect of the spinal cord, and a ventral 

 nerve arising by numerous separate fibres : the 

 dorsal nerves supply the skin and the trans- 

 verse muscles and are therefore both sensory 

 and motor, the ventral nerves are purely 

 motor, supplying the myomeres. 



Sensory Organs. At the level of the 

 anterior end of the brain is a narrow ciliated 

 depression, the olfactory pit (Fig. 702, olf.) 

 opening externally on the left side of the 

 snout and connected at its lower end with 

 the median olfactory lobe. This structure 

 is supposed to be an organ of smell : in the 

 larva its cavity is in direct communication 

 with the neuroccele through an aperture, the 

 neuropore (np.), which becomes closed in the 

 adult. There is some reason for thinking 

 that the olfactory pit answers to the hypo- 

 physis or pituitary 'body of Urochorda and 

 Craniata (pp. 17 and 96). 



The organ of sight is an unpaired pigment 

 spot (e) in the front wall of the brain : it is 

 therefore a median cerebral eye. There is no 

 lens or other accessory apparatus. Smaller 

 pigment spots occur in the spinal cord 

 throughout the greater part of its length, 

 below the neuroccele. There is no trace of 

 auditory organ. A peculiar structure, the 

 groove of Hatschek, on the roof of the oral 

 hood, is supposed to have a sensory function, 

 and may be an organ of taste. Lastly, the sensory cells on the 

 buccal cirri give those organs an important tactile function. 



Reproductive Organs.- -The sexes are separate, but there is 

 no distinction, apart from the organs of reproduction, between male 

 and female. The gonads (Fig. 698, gon. t Figs. 697, A, and 699, g.) 

 are about twenty-six pairs of pouches arranged metamerically 

 along the body-wall, and projecting into the atrium so as largely 

 to fill up its cavity. The inner or mesial face of each pouch is 



VOL. II E 



FIG. 703. Amphioxus 

 lanceolatus. An- 

 terior portion of neuron 

 from above, showing 

 nerves. (From Willey, 

 after Schneider.) 



