TYPE PBOTOCHORDATA. 615 



forming the aqueduct of Sylvius of tlie mid brain (2), while 

 behind this again an expansion of the dorsal portion of the 

 dorsal fissure forms a fossa rhomboidalis (3) similar to that of 

 the Vertebrate hind-brain, the resemblance to the embryonic 

 Yertebrate brain being thus very marked — a resemblance 

 ■which is increased by the occurrence of a funnel-like extension 

 {N) of the anterior ventricle towards the dorsal surface of the 

 body, where it abuts upon a ciliated depression of the ecto- 

 derm. This is the remains of a communication of the ventricle 

 with the exterior which exists in the embryo, and has been 

 compared with the hypophysis of the Vertebrate brain, the 

 difference of its position in the latter being due to the flexion 

 of the brain round the anterior end of the notochord. 



From the brain region of the nerve-tube three pairs of 

 nerves are given off (Fig. 281), the first and second of which 

 come from the dorsal portion of the brain, while the third 

 pair on each side is double, consisting of a root arising from 

 the dorsal surface of the brain and another arising from the 

 ventral side, a condition which is repeated in the succeeding 

 metamerically-arranged nerves. The dorsal and ventral roots 

 never unite to form a common trunk as in the Vertebrata, nor _ 

 do the dorsal nerves bear a ganglion, but nevertheless they 

 are sensory in function, while the ventral nerves are motor, 

 supplying only the musculature of the body. 



Of sense-organs the papillae upon the cirrhi and the 

 ciliated depression in connection with the hypophysis have 

 been already mentioned, the latter, on somewhat insufficient 

 grounds, having be^n supposed to be olfactory in function. 

 In addition to these a pigment-spot, which may represent 

 an exceedingly simple eye, occurs upon the anterior end of 

 the brain, and in the roof of the oral cavity a patch of cells 

 occurs, surrounding a depression lined with columnar cells 

 bearing long refractive hairs. This last structure seems to 

 be a sense-organ of some kind, but its exact function is un- 

 known. 



Different structures have from time to time been con- 

 sidered excretory organs. In the first place an excretory 

 function has been assigned to a ciliated tube lying in the wall 

 of the oral cavity on the left side and communicating by a 



