94 TROCEEDINGS OF THE AMKKICAN ACADEMY 



described in connection with those organs. Tlie muscles for extruding 

 the l)ristles do not differ from those described for related forms. In- 

 stances have been found, both in the living animal and in sections, of 

 fibres running from a dorsal to a ventral bristle sac on the same side, 

 and servinir, doubtless, to retract the bristles. 



The muscular fibres of both the annular and longitudinal layers are 

 spindle-shaped and flattened. A few of them contain median swell- 

 iii2fs with larire, cfranular central nuclei and nucleoli, while others 

 have a small nucleus on the free edge, and therefore approach the 

 muscular fibres of Nematodes in form. The cells of the bristle sacs, 

 partitions, and all the special muscles, consist of a large central part 

 containing a granular nucleus with its nucleolus, and of two or more 

 processes arising from the central part. 



The pecidiar muscular cells found inside the digitiform appendages 

 of the pavilion are described in comiection with that organ. 



Connective -Tissue Cells. 



Connective-tissue cells (PI. II. figs. 1 1 . 1 G, 1 7, cV.) are found in great 

 abundance everywhere in the body cavity. They are of various sizes, 

 being sometimes very large, and have deeply stainable, granular contents 

 and nuclei with nucleoli. They vary in shape, but always have one 

 or more long processes which are attached to surrounding parts. They 

 are found in the greatest number above the pharynx, where they form 

 an almost continuous mass, which connects the pharynx to the body 

 walls above, thus helping to support it. These cells also line the 

 body walls internally, and take part in the formation of the partitions. 

 Many of them connect the alimentary canal and the nervous system 

 to the body walls and to each other ; and, in fact, they bind together 

 all the internal organs. They form the ext(M*nal covering (neuri- 

 lemma) of the nervous system, and also pass in between the ganglionic 

 cells and the fibrous portion of the nervous substance, thus separating 

 them. Whether or not they form the tissue that separates the nerve 

 fibres from one another I have been unable to determine. 



The Alimentary Ccrnal. 



The alimentary canal is a simple tube running through the axis 

 of the body cavity, and held in place by numerous museular fibres 

 and connective-tissue cells from surrounding parts, in addition to the 

 dissepiments. Its walls are composed of three layers: an internal 

 cuticula, a layer of ciliated epithelial cells, and a membrana proi)ria. 

 To these may be added a very imperfect layer of annular muscular 



