skeleton 



pharyngeal pouch 



oldest 



tongue bar 



youngest 



Figure 1-12. Development of tongue bars in the enteropneust. (After 

 Dawydoff, 1948) 



strand with several central cavities in it. The dorsal and 

 ventral cords are joined by a plexus of connectives around 

 the mouth tube in the posterior part of the collar. There is 

 an anterior nerve ring in the base of the protosome which 

 connects with the dorsal cord and from which the strands 

 of the proboscis plexus extend. Giant cells are present, 

 particularly in the collar region. 



The reproductive system consists of numerous sac-like 

 gonads, each of which opens separately to the surface. These 

 are scattered along much of the length of the pharynx or of 

 the metasome in some species. The body wall may be drawn 

 out as lateral wings in which the gonads are located. The 

 pores of the gonads are scattered over the dorsal surface of 

 the wing. 



There is a rather elaborate circulatory system of channels, 

 of which only a part are lined. There are dorsal and ventral 

 lined and contractile vessels. The blood moves forward in 

 the dorsal vessel. In the region of the collar, this vessel lies 

 deeper in the body and is enclosed on either side by a 

 perihemal space — anterior diverticula of the nietacoels. At 

 the anterior end of the collar, the dorsal vessel receives 

 bilateral vessels fi-om the sides of the protosome, then expands 

 as the central sinus, or heart. 



The central sinus lies dorsal to the stomochord and is 

 overlaid by the pericardial cavity, which is contractile. From 

 the central sinus, blood passes forward into a glomerulus 

 associated with the tip of the stomochord. Here waste ma- 

 terials are thought to be filtered from the blood into the pro- 

 tocoel, which is filled with sea water. This sea water is ex- 

 changed with the exterior water by the action of the ciliated 

 funnel or funnels opening dorsally in the constriction be- 

 tween protosome and mesosome. 



From the glomerulus (Figure 1-11 A) blood passes into 

 dorsal and ventral protosome vessels and into bilateral cir- 

 cumenteral channels. These last pass to either side of the 

 buccal tube to the ventral channel, through which the blood 

 moves posteriorly. There is a plexus of channels in the collar 

 region both in the body wall and the buccal wall. This net- 

 work drains through a ring vessel in the collar-trunk septum 

 into the dorsal vessel. The ventral vessel supplies the pharyn- 

 geal region by way of the branchial vessels, the hepatic 

 diverticula, the gut generally, and the body wall. The region 



of the gonads may be drained by lateral vessels, which mid- 

 way along their length connect with the dorsal vessel. The 

 blood is colorless and contains a few epithelial cells. 



Pterobranchs Rhabdopleura and Cephatodiscus are the two 

 genera of this group that are usually described. The third, 

 Atubaria, is known from a single collection made in Japanese 

 waters. These small animals, 1 to 5 mm in length, were first 

 collected by the Challenger Expedition in 1876. Rhabdopleura 

 is colonial, Cephalodiscus social, and Atubana solitary. The 

 first two live in a secreted coenecium (not chitinous) con- 

 sisting of branching tubes or irregular masses with tubular 

 pockets and external decoration in the form of filaments or 

 fibers (Figure 1-13). The tubes o{ Rhabdopleura are formed 

 of small ring-like units (Figure 1-14). All the tubes are 

 joined basally — as all of the individuals in the colony are — 

 by a black stolon. In its structure it has been compared to 

 that observed in Graptolites, but this comparison has been 

 rejected by Hyman (1959). 



The body of these organisms is covered with a ciliated 

 epidermis (some parts may lack cilia) and is divided into 

 the same three segments observed in the enteropneusts: pro- 

 tosome, collar (mesosome), and metasome. The metasome 

 is divided into a visceral sac and a stalk. The protosome is 

 a flattened, disc-like structure, and its lateral margins are 

 notched. A pigmented strand of epithelium extends from one 

 notch to the other below the central thickened and glandular 

 area of the ventral surface of the protosome. 



The collar bears a lophophore made up of a series of arms 

 fused at the base: four to nine pairs in different species of 

 Cephalodiscus, four pairs in Alubaria, and one pair in Rhabdo- 

 pleura. The arms have two rows of tentacles and may be 

 tipped by a glandular knob. An oral lamella leads from the 



animal extended from cavity 



coenecium 



RHABDOPLEURA CEPHALODISCUS 



Figure 1-13. General appearance of pterobranchs and their secreted, 

 common, external skeleton or coenecium. 



14 • CHORDATES, PROTOCHORDATES, AND THEIR RELATIONSHIPS 



