CHORD AT A 427 



have been compared to the "brown tubes" of Amphioxus. 

 Underlying the epidermis is a well-marked layer of longitudinal 

 muscle fibres. The muscles of the trunk show no trace of 

 metameric repetition. 



The mouth opens ventrally between the base of the proboscis 

 and the anterior rim of the collar. It leads into a straight 

 alimentary canal. The anterior half of the digestive tube is 

 partially divided into a dorsal and ventral half by two lateral 

 horizontal folds. The gill-slits open into the dorsal half. 

 Behind the region of the gUl-slits the intestine is characterised 

 by a ciliated dorsal and ventral groove. In some species 

 paired hepatic caeca open at intervals into this region, and in 

 at least one species these diverticula are described as opening 

 on to the exterior. A median dorsal and ventral mesentery 

 support the intestine. 



In the region of the collar the alimentary canal gives off 

 a diverticulum, which grows forward and supports the pro- 

 boscis. The cells of this diverticulum become vacuolated, and 

 assume an appearance which is common in the notochordal 

 tissue of higher animals. The lumen of the diverticulum 

 disappears, except at its base, and the whole structure, which 

 extends through but a short region of the body, has been 

 regarded as a notochord. This interpretation is supported 

 both by its mode of origin and its structure. On the other 

 hand, the chief blood-vessel is dorsal to it, instead of being 

 ventral to it, as in the higher Chordata. 



The paired gill-slits are visible externally as small pores 

 situated in the dorso-lateral grooves of the anterior portion of 

 the trunk. Each gill -slit opens internally into the dorsal 

 thick-walled section of the alimentary canal. They increase 

 in number throughout life, new sHts arising behind those 

 already existing. When they first appear they are circular in 

 outline, but, as is the case in Amphioxus, a tongue-like process 

 grows down from the dorsal surface and reduces the opening 

 to a U-shaped aperture. Between neighbouring gill-slits and 

 in the tongue-shaped process there is a skeletal rod, the whole 

 forming a branchial skeleton comparable with that of Amphi- 

 oxus. The gill bars receive a supply of blood from the dorsal 

 blood-vessel, and water passes through the gill-slits from the 



