THE PHYLA HEMICHORDATA AND ECHINODERMATA 



363 



merous pairs of gill slits open externally near the mid-dorsal line. The 

 middle part of the trunk contains the gonads, which are gray in the 

 female and yellow in the male. The posterior region contains only 

 the posterior part of the intestine and tapers gradually to the anus. 



Although each gill slit first appears as a simple slit, later in de- 

 velopment the internal aperture, the opening into the pharynx, becomes 

 U-shaped (Fig. 19.4). The fleshy tongue bar that grows down from the 

 dorsal margin is primarily a respiratory organ, and contains a capillary 

 network in which blood is oxygenated as it passes from the ventral blood 

 vessel to the dorsal blood vessel. 



Blood is carried forward in a dorsal vessel of the trunk and collar 

 to the heart, which lies in the proboscis (Fig. 19.1). It is then pumped 

 through the glomerulus (Fig. 19.3), a tortuous knot of vessels projecting 

 into the coelomic cavity of the proboscis, and passes posteriorly through 

 a ventral vessel. The coelomic epithelium covering the glomerulus is 

 glandular, and waste products are believed to be removed from the blood 

 at this point. The waste is excreted into the coelom and flushed out 

 with the sea water as the cavity is filled and emptied. Branches from 

 the ventral vessel in the trunk lead not only to the gills, but also to the 

 gonads, intestine and body wall. Collecting vessels from these organs 

 return all blood to the dorsal vessel where it is mixed as it passes forward 

 again. All of the major vessels are contractile. The stomochord (Fig. 

 19.3) is an outgrowth of the pharynx that extends into the proboscis. 

 The cells of this diverticulum are large and vacuolated, resembling the 

 cells of the chordate notochord. On the ventral surface of the stomo- 

 chord the mesoderm secretes a chitinous plate which together with the 

 stomochord supports and stiffens the proboscis. A notochord-like tissue 

 is also found along the ventral margin of the intestine in some enterop- 

 neusts. 



The nervous system is very poorly centralized and is more primitive 

 in most respects than that of the flatworms. The proboscis is underlaid 

 with a thin, continuous layer of neural tissue. Most of the collar lacks 

 this layer, but dorsally a longitudinal strip of ectoderm constricts off to 

 form a tubular collar nerve. The trunk has a layer of neural tissue 

 similar to that of the proboscis, and in addition the nerve fibers tend 

 to concentrate dorsally and ventrally to form longitudinal nerves. The 

 collar nerve appears to function primarily as a pathway for nerve fibers 



Openings to cxt(z,rior 



ue 



TonO^ 



■Openings intopharyrix- 



Figure 19.4. Diagram showing how simple gill slits (left) become U-shaped (right) 

 by the doungrowth of tongue-bars from the roof of each slit. The external opening 

 remains simple. 



