454 PHORON1S 



At the base of the two spirals of the tentacular crown lie two 

 ciliated pits, regarded by Caldwell and M'Intosh 1 as sensory 

 organs, but Benham looks upon them as glandular in structure 

 and function. Perhaps they secrete the substance from which 

 the tubes are formed. 



The skin is covered by a delicate cuticle secreted by the 

 underlying epidermis ; within the latter is a well-marked base- 

 ment membrane, and beneath this a layer of circular muscle 

 fibres ; these surround a layer of longitudinally -arranged fibres, 

 which do not form a continuous sheet but are arranged in 

 bundles. In both layers the fibres are unstriated. The longi- 

 tudinal fibres are covered on their inner side by a layer of flat 

 pavement cells, which line the general cavity of the body. 



This space, the body-cavity, is divided into two parts by the 

 presence of a diaphragm or septum which runs across from one 

 side of the body to the other about the level of the ridge bearing 

 the tentacular crown. The anterior space is continuous with the 

 cavities of the tentacles and of the epistome. The partition is 

 pierced by the blood-vessels and the oesophagus, but the rest of 

 the alimentary canal, including the anus, the kidneys, and the 

 reproductive organs, all lie in the posterior half of the body-cavity 

 behind the diaphragm. This portion of the body-cavity is further 

 subdivided by the presence of three longitudinal mesenteries 

 supporting the alimentary canal and running between it and the 

 body- wall. One of these mesenteries runs along the outside of 

 the alimentary canal throughout its whole length, attaching both 

 the descending and ascending limbs of the U-shaped tube to the 

 body-wall. The other two are lateral mesenteries, which pass 

 from the body- wall to the sides of the oesophagus. These 

 mesenteries therefore divide the body-cavity into three spaces 

 one in which the rectum lies, which may be called the rectal, 

 and two lateral ; owing to the fact that the lateral mesenteries 

 end before they reach the bend of the alimentary canal, the 

 three chambers are in free communication one with another. 

 The body -cavity is further traversed by irregular strands of 

 tissue which run from the body-wall to the various organs. It 

 contains a corpusculated fluid. 



The alimentary canal (Fig. 230) consists of a U-shaped tube 



1 Challenger Reports, vol. xxvii. 1888 ; and Proc. Roy, Soc. Edirib. vol. xi. 1882, 

 p. 211. 



