314 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



In the wall of the end gut the connection of the fibrillse with the cells is 

 easily seen. In the gelatinous ground substance, which appears more or less finely 

 granular, lie spindle and star shaped cells, the processes from which radiate in 

 all directions through it, uniting with the processes of other cells at various places. 

 Wlien the fibrillse run parallel to each other the tissue acquires, if they are suffi- 

 ciently numerous, a cartilaginous consistency, as is more or less evident in the 

 inner layers of the cutis. Amoeboid cells of all shapes are present and are often 

 more or less grouped. 



In the arms of Heliometra glacialis there are found cells 0.01 mm. in diameter 

 which show one or several processes, or may be more or less spherical. A spherical 

 nucleus 0.002 mm. in diameter lies in the granular cytoplasm. 



In Antedon the cell substance does not stain, but a reticulation can be made 

 out in the meshes of which lies the deeply stained nucleus. About the dorsal 

 portion of the mesodermal nervous system these cells are abundant. 



In Neocomatella pulchella on the dorsal and lateral surfaces of the arms and also 

 beneath the epithelium of the anal tube prominent cell clusters appear in the form 

 of more or less egg-shaped structures. These cell groups, each consisting of five 

 or more cells, occur immediately beneath the external epithelium. The individual 

 cells are bladder like, consisting of a thin membrane in which the cell substance is 

 inclosed. The cells are filled with a watery transparent fluid which is not stainable. 

 A spherical nucleus lies more or less centrally, surrounded by a small amount of 

 plasma, which, in the form of pseudopodia-like threads, stretches to the wall, 

 holding the nucleus in place. The walls of occasional cells are often strongly 

 pleated. 



Such cell groups are a prominent feature of the walls of the pinnules in all 

 the species of Comasteridse which have been studied, but they have never been 

 found in any endocyclic form. 



It has been supposed that their function is to antagonize the ventral muscles. 



Pigment cells, containing free pigment granules, are abundant elements in 

 the mesoderm of the cutis and occur also widely distributed throughout the body. 



VISCERAL MASS. 



In Antedon the body wall of the visceral mass is everywhere uniform except 

 at the ambulacral furrows, beneath which the water vessels and other organs lie. 

 In mature animals the thiclaiess of this wall is at the most 0.1 mm. 



It is composed of four layers: (1) The general body epithelium, composed of a 

 single layer of cells; (2) the cutis; (3) the connective tissue layer; and (4) the 

 epithelial lining of the body cavity. 



The external epithelium has secreted no cuticle, and is scarcely to be differen- 

 tiated from the connective tissue layer. The small size of the component cells often 

 renders them difficult to demonstrate. There are no cell borders, but the presence 

 of nuclei lying closely together in an irregular layer indicates the presence of 

 an epithelium. 



