1912] The Ottawa Naturalist. 47 



fields of their origins were on the oral faces of the marginals. 

 These fields were outside of a highly specialized area, were 

 therefore more diffused and their limits are not recognizable 

 As all the adambulacrals were free to move in a direction 

 perpendicular to the oral plane, the attachment of the epineural 

 adductors to them necessitated a series of ambulacral depressors 

 with origins on the aborad ambulacral edges of the marginals. 

 The adambulacral floor could be thus raised or lowered while 

 the epineurals w^ere closed. 



Plate II, fig. 5, shows also marked muscle pits on the aborad 

 surfaces of the seventh pair of adambulacrals of aim V, while 

 the photomicrographs made for fig. 7 of the same plate showed 

 but faint muscle pits on the aborad surfaces of the fifteenth pair 

 of adambulacrals. These pits represent the places of attach- 

 ment of one of a series of three adambulacral adductors for 

 each row. It is very evident that the muscles of the peristomial 

 ring could act as abductors of the older adambulacrals. 



On younger portions of the arm the orad aborad move- 

 ment of the adambulacrals (allowing the adambulacral jaw to 

 be advanced or retracted) was not permitted, as mav be seen 

 by an examination of the fourth and fifth arm marginals of arm 

 II (plate III, fig. 1). The ossicles were here so slightly attached 

 to the carbonized bed of the substratum that an attempt to find 

 ambulacrals and aboral plates resulted in the loss of the 13th 

 adambulacral of the lower row. Further attempt to develop 

 this locality was immediately abandoned but the accidental 

 removal of the single ossicle left a perfectly fresh surface, showing 

 the semi-cylindrical groove in which movement perpendicular 

 to the oral plane was allowed while movement along the ray 

 was prevented. The median vertical ridges on the ambulacral 

 faces of these younger marginals may be clearly seen. The 

 younger and weaker ambulacral adductors were thus protected 

 from the pull of the peristomial ring. The change in outline 

 of cross section of the prismatic flooring pieces, while passing 

 orad, is indicative also of change in function. The very marked 

 increase in curvature of what were once prism angles was in 

 part due to a demand for larger fields for origin and insertion of 

 muscles other than those already mentioned. 



Food Capture. 



That the open epineurals of ciliated food grooves had occa- 

 sionally the chance to capture animals somewhat larger than the 

 organisms making up the mass of the food, cannot be doubted. 

 When our primitive stellerid abandoned the fixed habit and 

 began to find a more abundant food supplv in the ooze of the 



