78 ARTHUR E. SHIPLEY. 



are visible as local thickenings of the cuticle, and are often 

 loaded with a bright yellow-brown pigment. 



The body of the papilla has the form of a double cup, as if 

 it had been formed by the invagination of a spherical out- 

 growth of the general ectoderm. The outer layer of the cup 

 is composed of flattened cells, which aie continuous with those 

 of the general ectoderm at the base of the papilla, and with 

 those of the inner cup at its apex. The inner layer of the cup 

 consists of large cells, loaded with granules of a bright yellow 

 substance, so that the remains of their protoplasm are seen as 

 slender strings of stained material, separating masses of the 

 yellow formed material. This inner cup contains a small 

 cavity, which communicates with the exterior by the pore at 

 the apex of the papillae. Between the two cups is a cavity, 

 continuous with the subepidermal system of spaces above 

 mentioned. 



In the absence of a detailed knowledge of the habits of the 

 living Phymosoma it would be rash to assign any function to 

 these very curious organs, but it seems not improbable that 

 the secretion they produce may assist in softening the coral 

 rock in which the animals form long tubular passages. 



General Anatomy. 



The arrangement of the internal organs is shown in fig. 3 

 which represents a Phymosoma cut open longitudinally and 

 the body wall turned back to expose the viscera. The intro- 

 vert is invaginated to almost its full extent, the true anterior 

 end of the body being at the point where the sense-pits lie. 



The longitudinal and circular muscles of the skin have been 

 omitted for the sake of clearness ; a detailed description of them 

 is given below. 



The retractors of the introvert are four in number. They 

 fuse round the first half of the oesophagus forming a muscular 

 tube, and then separate into a dorsal and a vential pair. The 

 former are much the shorter pair; between them lies the 

 dorsal blood-vessel, whilst the ventral pair have at their base 

 the generative ridge and between them the nerve-cord. The 



