SPATANGINA. 95 



differentiated as the subanal plastron (p. 93). The lateroanal fasciole, when 

 fully developed, is a band which runs from the side of the peripetalous fasciole 

 downward and backward under the periproct and upwards and forwards to the 

 other side of the peripetalous fasciole; it is seldom as complete as this for the 

 connection with the peripetalous fasciole on each side is often wanting, and the 

 lateral portions may be reduced to such an extent that only the piece under 

 the periproct is left; no other fasciole shows as great specific and individual 

 diversity as does this. Mortensen discovered that it is in reality the posterior 

 portion of the original peripetalous fasciole and that the present posterior part 

 of that fasciole (at least in many of the Hemiasteridae) is a cross-band of second- 

 ary origin. Obviously then there is no homology between the subanal fasciole 

 and the lateroanal. The anal fasciole is made up of bands on each side of the 

 periproct and usually, if not always, these bands appear as branches of the 

 subanal fasciole; they may then be called simply the anal branches. The 

 name anal fasciole is also used sometimes for that part of the lateroanal which 

 lies in the immediate vicinity of the periproct, particularly if its lateral connec- 

 tions with the peripetalous fasciole have disappeared. 



The Spines, Pedicellariae, Sphaeridia, and Spicules. 



In all Spatangina, there are spines carried on the plates of the test, but 

 their size, number, and distribution is a matter of the greatest diversity. We 

 can recognize primary, secondary, and miliary spines, with their corresponding 

 tubercles, but there is no sharp line between these groups and a spine which 

 would be a primary in one genus might be only a secondary in another. The 

 terms are purely descriptive for convenience and probably have no morphologi- 

 cal significance. As a rule, the spines are solid, bluntly rounded, or pointed 

 at the tip. But in some genera the miliaries are multiscalariform (t. e. made up 

 of a number of rods connected by cross-bars like the rounds of a ladder) as in 

 many of the clypeastroids. Secondaries are oftentimes flattened at the tip, 

 thus becoming spatulate, and these may also be curved. This curved spatu- 

 late form is also quite characteristic of the primaries of the oral surface, especially 

 those on the posterior part of the sternum. Secondaries bordering the petals, 

 especially if these are sunken, and those surrounding the periproct, are usually 

 the largest and most conspicuous secondaries on the test. The largest prima- 

 ries are as a rule on the oral surface, particularly in interambulacra 1 and 4. 

 where those bordering the ambulacra I and V are of maximum size. Similar 



