244 W. R. Coe — Development of the 



boundaries (Fig. 6) can be made out only after treatment with suita- 

 ble reagents. 



In the lappets the cells become much thicker and are correspond- 

 ingly more numerous. On the free edges of these oi'gans they are 

 closely pressed together and become highly columnar. All the 

 ectoderm cells are covered with cilia, the length of which corre- 

 sponds more or less closely with the thickness of the cell oti which 

 they are borne. Thus the very thin cells of the dorsal surface have 

 very short, scattered cilia, but as the columnar cells at the edges of 

 the lappets are approached, the cilia become correspondingly longer 

 and more closely packed together. 



The apical j^late is sharply marked, and is usually provided with a 

 single flagellum nearly as long as the body. There are commonly 

 several shorter and more slender flagella in addition to the primary 

 one (Fig. 6, PI. xxxir). 



The mouth is highly distensible, and provided with thickened lips 

 and buccal folds as described below. It is triangular in form and is 

 situated slightly in front of the middle of the subumbrellar surface. 



The esophagus is remarkably spacious and extends well upwards 

 towards the apical plate. The intestine is comparatively small. It 

 usually contains a quantity of small particles — probably food-ma- 

 terial — which are kept in rapid motion by the long cilia lining the 

 cavit3^ 



The food of the pilidium probably consists of small pelagic larvfe, 

 infusorians, diatoms, etc. — in short, of any minute organisms which 

 can be carried into the intestine b}' the ciliary motion. 



On the sides of the body the network of highly refractive muscn- 

 lar fibers with their clear nuclei are always conspicuous. They will 

 be described in detail below. 



Near the borders of the lappets and below the intestine a large 

 number of mesenchyme cells are formed. Some of these are con- 

 spicuous from their decidedly yellowish color. Clusters of such yel- 

 lowish cells doubtless correspond to the " golden-yellow spots around 

 the margins" of the pilidium figured by Verrill (30, Fig. 6, PI. 39). 

 This is no evidence that the species are identical, however, for a 

 brown or yellow color is found in such cells in several other species. 



The pilidium of C. leidyi (Fig. 5, PI. xxxii) differs from that 

 described above mainly in the greater extent to which the lappets 

 ai*e developed in the former. In general form it is broader at the 

 base, and not bO much swollen above as in M. cceca. The size and 

 general ajjpearance otherwise are so closely similar that in certain 



