i64 ARCHICCELOMATA. 



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 hepatic 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. 



In the collar there are paired coelomic 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 

 perihcemal) and two lateral (or peripharyngeal) processes. 

 The coelom 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 consiiluent layers of the dorsal and ventral 



