348 CHORDATE ANATOMY 



NERVOUS SYSTEM OF CHORDATES 



Hemichordates. Hemichordates have a dorsal tubular nerve cord 

 limited to the collar region, but a ventral invertebrate type of nerve 

 cord in the gill and trunk region. (Fig. 4) The tubular portion of the 

 dorsal cord remains open at both ends throughout life. It contains neuro- 

 sensory cells of a primitive type and some giant ganglion cells, is sur- 

 rounded by an outer fibrous layer as in vertebrates, and continues forward 

 into the proboscis and backwards into the body as a non-tubular strand 

 of nervous tissue. A circumesophageal ring connects this dorsal cord with 

 a ventral median strand. The ventral nerve strand is simply a local thick- 

 ening of a layer of nerve fibers which forms a continuous network beneath 

 the skin. 



The association in hemichordates of invertebrate and vertebrate 

 types of nervous system helps to bridge over the gulf between the two 

 groups. 



Urochordates. The nervous system of larval urochordates shows an 

 advance towards that of vertebrates, for the ventral invertebrate nervous 

 system has disappeared, and the nerve cord is tubular throughout. 

 Three divisions may be distinguished: an expanded anterior brain or 

 sense vesicle which encloses an unpaired eye and static organ; a short 

 trunk portion; and, behind, a slender cord which in the larva extends 

 into the tail. Paired nerves connect the cord with the caudal muscle. 

 The brain is anterior to the notochord, and has therefore been considered 

 as the homologue of the forebrain of vertebrates. (Fig. 2, A) 



In most urochordates, the tail is lost during metamorphosis, and with 

 it the associated nerves, so that only in Appendicularia do these persist 

 throughout life. 



Cephalochordates. Cephalochordates have a nervous system with 

 many vertebrate characteristics. The cord extends through the entire 

 length of the body as a tube with a slit-like lumen which is expanded 

 anteriorly in the region of the so-called brain. It also resembles that of 

 vertebrates in its origin from a thickened placode of ectoderm on the 

 dorsal side of the embryo. The neuropore persists in the larva, but 

 closes in the adult animal to form the so-called olfactory pit. (Fig. 2 , B) 



Two divisions of the brain are recognized, an anterior prosencephalon 

 and a posterior deuterencephalon. The prosencephalon is lined with 

 cihated columnar epitheUum which shows little if any nervous differentia- 

 tion. In its anterior wall is a pigment spot which, with scant justification, 

 is called an eye. From the prosencephalon a pair of sensory nerves, the 

 terminal nerves, extend forward towards the snout. The posterior 

 boundary of the brain is marked by a cluster of ciliated sense cells, the 

 infundibular organ. 



