544 PHORONIDEA. 



the anus, the whole of the aboral extension of the body being 

 ventral. The tentacles possess within the epidermis a ring of skeletal 

 tissue of mesoblastic origin, and contain a blood vessel and an 

 extension of the oral coelom. The body-wall consists of an ecto- 

 dermal epithelium, a basement membrane, the usual two layers of 

 muscle, and internally the coelomic epithelium. 



The nervous system lies in the skin immediately within the 

 epidermis, and outside the basement membrane. There is a special 

 concentration of it round the niouth in the form of a circumoral 

 ring (A 7 ), which is enlarged on the dorsal side between the mouth 

 and the anus into what may be called a dorsal or supra-oesophageal 

 ganglion. This ring follows the curve of the lophophore near the 

 base of the outer tentacles, and gives off nerves to the tentacles 

 and kidneys. There are also two tubes lying in the skin, and 

 extending aboralwards from the nerve ring along the insertion of 

 the lateral mesenteries. These are probably large nerve-fibres com- 

 parable to the giant-fibres of earthworms, etc. Some punctated 

 nervous tissue lies internally to the tube of the left side. 



There are no organs of special sense. 



On the dorsal side of the base of the inner series of tentacles are 

 two ciliated pits, which have been interpreted by some observers as 

 glandular structures, by others as sense-organs (gl). 



The alimentary canal is a U-shaped structure occupying the aboral 

 extension of the body. It presents four regions the oesophagus ; 

 the stomach (st), at the base of the proximal limb of the U ; the 

 intestine, or second stomach (int) ; and the rectum, which leads to 

 the anus. The walls of the stomach are glandular. 



The perivisceral cavity is entirely coelomic ; it is divided at 

 the level of the lophophore by a transverse septum (s) into a 

 small oral section, which is continued into the epistome and 

 tentacles, and a larger posterior (aboral) part in relation with 

 the alimentary and other viscera. This septum is perforated by 

 the oesophagus, but not by the rectum, the anus being placed 

 aborally to it. 



The posterior (aboral) section is further subdivided by longitudinal 

 mesenteries, which run from the gut-wall to the body-wall. One of 

 these is a median ventral mesentery (a and c) attaching the outside 

 of both descending and ascending limbs of the alimentary canal to 

 the body-wall. Besides this there are two lateral mesenteries passing 

 from the oesophagus and stomach to the body-wall (Fig. 438, b). The 

 body-cavity is thus divided into three chambers a rectal chamber 



