CHORDATA 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 gill-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 slits 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 
