478 COMPARATIVE ANATOMY 



The association in hemichordates of invertebrate and vertebrate 

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

 groups. (Fig. 532) 



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. 538, 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. 538,5) 



Two divisions of the brain are recognized, an anterior prosencephalon 

 and a posterior deuterencephalon. The prosencephalon is lined with 

 ciliated columnar epithelium 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 

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

 infundibular organ. 



The deuterencephalon is possibly homologous with the mid- and 

 hindbrain of vertebrates. It differs from the spinal cord in having in its 

 dorsal wall large neurosensory cells known as cells of Joseph. Two 

 paired dorsal sensory nerves — numbered II and III — connect with the 

 deuterencephalon. The first pair of motor nerves connect with its 

 ventral wall. (Fig. 436) 



In amphioxus dorsal and ventral nerves alternate with one another 

 throughout the length of the body. Except the two anterior pairs, 

 which are wholly sensory, the dorsal nerves of Amphioxus are mixed in 

 function. They extend between the myotomes to the skin, where they 

 divide into dorsal and ventral rami. Ventral nerves, on the other hand, 

 pass from the cord directly to the myotomes opposite. Consequently, 

 in cephalochordates, dorsal and ventral nerves do not unite. The 



