XIII 



PHYLUM CHORDATA 



55 



extremit}^ of the bi-ain, the second from its dorsal region: they 

 are both distributed to the snout, their branch(;s being pro- 

 vided towards then- extremities with numerous ganglia containing 

 nerve-cells. The s|)ii\al 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. 748). 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. 747, olf.) 

 opening externally on the left side of the 

 snout and connected at its lower end with 

 the medial olfactory lobe. This structure 

 is supposed to be an organ of smell : in the 

 larva its cavity is in direct communication 

 with the neurocoele through an aperture, the 

 neuropore (i^).), which becomes closed in the 

 adult. There is some reason for thinking 

 that the olfactory pit answers to the ht/jw- 

 physis or pituitary hody of Craniata (see 

 Section II.). 



The organ of sight is an unpaired pigment- 

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

 therefore a median cerebral eye. There is no 

 lens or other accessory appara^. is. Smaller 

 pigment-spots occur in the spinal cord 

 throughout the greater part of its length 

 below the neurocoele ; these are said to re- 

 present groups of minute, rudimentary, light- 

 perceiving organs. 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, but this is very doubtful. Lastly, the 

 sensory cells on the cirri of the oral hood 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. 743, gon.. Figs. 742, A, and 744, g.) 

 are about twenty-six pairs of pouches arranged metamerically 

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





J 



P"io. 748.— Amphioxus 

 lanceolatUS. An- 



terior purtiuii of neuron 

 from above, showing 

 nerves. (From Willey, 

 after Schneider.) 



