ON PHTMOSOMA VAEIANS. 9 



shaped aad split up into bundles of fibres, which pass into the 

 adjacent longitudinal bundles. 



A special muscle accompanies the nervous system on each 

 side (fig. 29), and is described in connection with the nerve- 

 cord. Its purpose is probably to regulate the movements of 

 this important organ during the eversion or retraction of the 

 introvert. 



The spindle-muscle and the intrinsic muscles of the ali- 

 mentary canal are described with the digestive organs, and 

 the intrinsic muscles of nephridia with the account of these 

 organs. 



Except along the generative ridge, the body wall is lined by 

 a layer of flat epithelial cells, which is never ciliated, in this 

 respect differing from that of Sipunculus. 



The Skeletal Tissue. 



A curious form of tissue is found in the collar and the ten- 

 tacular crown of Phymosoma. As it seems to subserve the 

 purpose of supporting and stifi'ening the collar and tentacles, 

 and as a support for the insertion of the retractor muscles, I 

 propose to call it the skeletal tissue. 



The cells composing this tissue are large rounded cells, which 

 lie close to one another, but are not so crowded as to become 

 hexagonal. The cell nucleus is large, and both it and the proto- 

 plasm of the cell stain deeply. Running across the cell, usually 

 in a radial direction, are a small number of wavy lines. 



This tissue forms a ring lying in the substance of the collar, 

 which it seems to stiff'en. The horseshoe-shaped blood-space 

 lies internal to this tissue, which is thicker at some parts, 

 and thus serves to break up the blood-space as indicated in 

 figs. 4 and 6. It also sends extensions into the tentacles, a 

 group of these skeletal cells being formed on both sides of the 

 tentacular nerve in each section of the tentacle (fig. 17). 



From the position of this skeletal ring in the collar it will be 

 readily understood that it is juSt in front of the invaginable 

 introvert, and consequently it affords a valuable hold for the 



