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of the gill-pouches, which also have internal openings 

 (gill-slits) in the side walls of the pharynx. The anus is 

 terminal, at the attenuated end of the trunk. The body 

 wall is muscular and contractile. The creature moves by 

 wriggling, and by means of its proboscis ; and it swallows 

 the mud, its food being the organic matter which the mud 

 contains. 



The ectoderm has a feltwork of nerve- fibrils ; a mid- 

 dorsal thickening of this, in the trunk, is the dorsal nerve- 

 cord, which is closely connected with a central nerve mass, 

 or neural tube, in the "collar." There is also a similarly 

 formed ventral nerve - cord. A nerve-ring (between the 

 " collar " and the trunk) connects the two nerve-cords. 



The gut or alimentary canal is a straight tube. The 

 first portion of the gut (within the " collar ") is the buccal 

 cavity, which has a dorsal tubular outgrowth with a tough 

 fibrous sheath ; this is the notochord, very short and re- 

 stricted to the proboscis into which it extends and which 

 it supports. The second portion of the gut is the pharynx ; 

 it has two distinct regions, an upper one with the internal 

 gill-slits in its walls, and a lower one along which the food 

 (mud) passes. Between the gill-slits there are skeletal 

 supports called primary bars, and each slit is divided by 

 a tongue-bar. The body cavity or coelome is developed as 

 five pouches from the embryonic enteron. The largest 

 of these are two (blind) spaces situated in the trunk through- 

 out its length ; the others are the two " collar " sacs, 

 which open into the first pair of gill-pouches, and the 

 proboscis sac, which opens to the exterior by one or by 

 two dorsal pores. On the inner wall of the proboscis sac 

 is the glomerulus, an excretory gland ; it is situated at 

 the front end of the notochord, above which is the peri- 

 cardium (a closed sac). 



Above the notochord, between it and the pericardium, 

 is the dorsal heart ; it is the anterior contractile portion 

 of the dorsal blood channel, along which the blood flows 

 forward from the body into the glomerulus. Thence the 

 blood passes indirectly into a ventral blood channel (below 

 the gut) in which the flow is backwards. 



The gonads or reproductive organs are derived, like the 

 muscles and connective tissue, from the coelomic walls, 

 and form a series of simple sac-like outgrowths in the 

 trunk coelome of each side, in and behind the gill-slit region. 

 The gonads open to the exterior by minute genital pores. 

 The sexes are separate. Development is either direct (e.g., 

 Dolichoglossus) or indirect (e.g., Balanoglossus) involving a 

 free-swimming Tornaria larva. 



