28 



VERRILL 



Fig. I. 



Diagrams generalized to illustrate progressive ontogenetic and phylogenetic development 

 of the plates and spines in the rays of starfishes {Asteriida). Lettering as follows; the 

 primary rows are shaded: 



md. Median dorsal or carinal row. m, Marginal rows, m', Intermarginal row. sm, 

 Superomarginal row. int, Inferomarginal row. ad, Adambulacral row. din, Dorsal-inter- 

 mediate, medio-latcral, or dorso-lateral rows, p or pa, Peractinal row of plates, sa, Sub- 

 actinal rows. These and the peractinals are the interactinal plates, collectively, o. The 

 ocular, apical, or terminal plate. 



i, The simplest or most primitive condition, common in the young; uncommon in the 

 adults, in which only the primary rows of plates are developed. 



The longitudinal area between ad and the next row is called the perambulacral lane or 

 channel; the space between the marginals (»() is called the intermarginal latie or channel; 

 the area next above the upper marginal is called the supramarginal lane or channel. These 

 " lanes " or " channels " are always crossed by transverse connective ossicles or by over- 

 lapping lobes of the plates, dividing them up into larger or smaller papular areas, and they 

 usually bear more or fewer of the larger pedicellarise in the Asteriidae. 



ii. The condition, more advanced, in which the peractinal row (/?) and one medio-lateral 

 row (din) are developed. A common condition in the Asteriinse. This is called the mono- 

 actinoplacid condition. 



iii, A more advanced condition in which an additional interactinal row Csa, subactinal) 

 and a second dorso-lateral or medio-lateral row idin) have appeared. 



iv, A more complex state, in which there are two subactinal rows (sa), and an inter- 

 marginal row (m'). 



This is a common condition, but more medio-lateral rows are often added, as well as more 

 subactinals, in such genera as Pisaster, Evasterias, etc. This is called the polyactinoplacid 

 condition. All these plates usually bear spines, one or more each. They may be easily 

 visible or they may be entirely concealed by a thick tough skin. The peractinal or sub- 

 actinal plates, when thin or small, may be destitute of spines and stand edgewise, so as to 

 show only the edge at the surface, or they may be so closely joined to the inferomarginals 

 as to require cleaning with potash to be seen. 



