ON A NEW SPECIES OF PHYMOSOMA. 113 



layers occurs, the longitudinal muscles fusing into a continuous 

 sheath, the circular muscles becoming broken up into bundles. 



The retractor muscles are two in number, a right and a left ; 

 they arise about the level of the junction of the anterior two 

 thirds with the posterior third of the body. They embrace the 

 oesophagus, forming a semicircular band of muscle-fibres which 

 are wanting only in the dorsal middle line where the heart lies. 



The heart is provided with very numerous csecal diverticula 

 (figs, 4 and 7). 



The external aperture of the kidneys lies on a level a little 

 behind that of the anus. 



Habitat : the lagoon, Bimini Island, the Bahamas. 



Part II. 

 The Papillae. 



The papillae of the skin are of two kinds, those on the body 

 and those on the introvert. In the middle of the trunk the 

 papillae have an oblong outline, and are arranged in very 

 regular rings (figs. 1 and 3) ; near the posterior end of the body, 

 and also at the base of the introvert, the papillae are so crowded 

 together as to lose their rectangular outline. These papillae 

 are of a dark brown colour, and in those regions where they are 

 crowded together the buff colour of the rest of the skin is 

 completely obliterated. The trunk papillae form only low 

 elevations above the general level of the skin ; each has a 

 central pore, surrounded by a number of brown horny plates, 

 which are modifications of the cuticle. These plates show a 

 faintly laminated structure ; they are represented in section in 

 figs. 11 and 12. 



Between these brown plates are placed a number of deeply 

 pigmented granules of a dark brown, almost black colour. 

 These give the dark brown colour to the papillae (figs. 9 and 12). 

 Neither the plates nor the pigment granules show any trace of 

 being connected with any special cells ; they seem to be modifi- 

 cations of the cuticle. The papillae, like those of Ph. varians, 

 are formed by the ectoderm-cells rising up and invaginating to 

 form a double cup. The outer wall of the cup is formed by 



