vessel passes between the myomeres and opens into the 

 myocoel, the cavity between the myomere and the body wall. 

 The ventral vessel passes down on the inner aspect of the 

 body wall and enters the cardinal vessel. Irregular branches 

 from the dorsal aorta also extend to the intestine. The dorsal 

 aorta e.xtends back into the tail as the caudal artery, and 

 blood is collected in the tail by a caudal vein. The vein ex- 

 tends forward to enter the subintestinal vessel and the pos- 

 terior cardinal of the right side. 



The blood of Amphioxus is without cells, and the vessels, 

 with the exception of the dorsal aorta, are without linings! 

 The subintestinal vein, hepatic vein, endostylar vessel and 

 Its bulbuli (tiny expansions), and the nephric glomerulae 

 are contractile. The term glomerulus refers to a knot, or in 

 this case a net, of small blood ves.sels. Such "knots" of small 

 arterial vessels are associated generally with excretory struc- 

 tures (Figure 1-4 D,E). 



The nervous system (Figures 1-2 to 1-4) consists of a dor- 

 sal nerve tube and paired segmental nerves. In this tube 

 are scattered photosensory cells, the cells of Hesse and 

 Joseph. Those of Joseph are distributed anteriorly and dor- 

 sally, those of Hesse along the inner face of the tube through- 

 out most of its length. The former are cells enclosed by a 

 cup of pigment granules. Giant cells (of Rhodes) are seg- 

 mentally arranged for much of the length of the tube, and 

 their fibers run ventrally along the cord, on the side opposite 

 the cell body. 



Anteriorly the cord is slightly expanded and its central 

 cavity enlarged to a ventricle. The ventral and dorsal 

 nerves of each segment are separate. The ventral root lies 



loop of gut. 



stolon 



opposite the myotome; the dorsal root passes between the 

 myotomes, posterior to the ventral root. The dorsal root of 

 one side is across from the ventral root of the other side. The 

 dorsal root is largely sensory but contains motor fibers for 

 the nonmyotomic muscles of the ventral part of the body 

 (the transverse muscle) and fibers for the gut wall (there is 

 a plexus, or nerve net, in the gut wall). The nerve fibers are 

 not myelinated, and sensory ganglia are lacking— the sen- 

 sory cell bodies lie close below the epidermis or are scattered 

 along the course of the nerve. 



The two most anterior nerves arise from the expanded 

 "brain." These serve the snout and lack ventral roots. Nerves 

 III to VII have dorsal and ventral roots, the latter serving 

 the myotomes. The dorsal nerve gives rise to superficial and 

 deep branches which, along with the deep branch of the 

 second nerve, give rise to superficial and deep plexuses in 

 the walls of the oral hood. The velum is innervated by 

 branches from the deep plexus of the left side. There is no 

 lateral-line system nor are there special sense organs such 

 as the eye, ear, or nose. 



Urochordata (Tunicates) 



The adult tunicate is usually an attached (sessU) organism; 

 a few species are planctonic (floating about), whereas othei-s 

 retain the larval form as adults. They may be solitary, 

 colonial, or compound (Figure 1-7). Clavelma or Ectemas- 

 cidia are solitary types of tubular or sac-like form. 



At the free end of the tunicate there is a mouth and an 

 atrial pore. The animal is enclosed in a tunic or secreted 



^ommon atrial opening for ring of individuals 



b^°'" muscle bonds 



mouth 



atrial opening 



endastyle/ , ' ovary 



' heart ' 



DOLIOLUM 



statocyst 



mouth. 



common atrial opening 

 for ring of individuals 



CLAVEMNA 



SYNOICUM 



APPENDICULARIA 



Figure 1 -7. Basic kinds of tunicates: solitary (C/ave/ino), colonial (Synoicum), and pelagic [DoMum, 

 Appendiculoria). A shows individual removed from colony, B. 



