164 ARCHICELOMATA. 
the pharyngeal wall into a stiff body. Hence the subneural 
gland serves to support the proboscis, and the pharyngeal 
wall with the mouth are permanently rigid and open. 
The posterior portion of the pharynx becomes almost 
divided into a dorsal and ventral part, each of which is 
ciliated. The pharyngeal clefts open into the dorsal por- 
tion, which is often called the respiratory part, the ventral 
being distinguished as the nutritive portion. Water, mud 
and nutritive particles pass in at the mouth through the 
pharynx. The water is said to pass out dorsally by the 
pharyngeal clefts The mud and food pass ventrally into 
the intestine in which digestion is effected. In many 
species numerous fepfatic glands open dorsally into the 
intestine. The intestine runs to the end of the trunk and 
terminates in the anus. 
The external segments of the body are found to corre- 
spond to cavities of the mesoderm. The proboscis has a 
single coelomic cavity, opening behind by a 
single or, in some cases, two proboscis-pores. 
The walls of the cavity form complex muscles for the 
movements of the proboscis. 
Colom. 
In the collar there are paired ccelomic cavities separated 
by dorsal and ventral mesenteries which suspend the 
pharynx. Each opens to the exterior by a collar-pore. 
The trunk also has a pair of cavities with dorsal and ventral 
mesenteries. The walls form a well-developed system of 
longitudinal muscles and paired gonads. The trunk-cavities 
are produced forwards into the collar by two dorsal (or 
perthemal) and’ two lateral (or seripharyngeal) processes. 
The ccelom is filled with a network of connective tissue. 
The nervous system consists of a central nerve-mass in 
the dorsal region of the collar, a nerve-ring 
round the hind end of the collar, and a median 
dorsal and median ventral nerve along the trunk. Nerves 
pass forwards on to the proboscis. 
Except in the dorsal collar-region, the nerves are still 
parts of the ectoderm and are connected in all directions 
by a diffuse nerve-plexus. 
The blood-vascular system consists of sinuses or vessels 
between the constituent layers of the dorsal and ventral 
Nervous. 
