ARRANGEMENT AND DEVELOPMENT OF PLATES. 139 



growth, but their view as expressed is quite the opposite of our conclu- 

 sions. This view was based on 3 specimens studied, all of which they 

 state have 6 columns of plates. The answer would be that while the num- 

 ber of columns apparently does increase with age, the full number may be 

 attained quite early in the life of the individual.* In such a case a con- 

 siderable increase in size may take place without the addition of any 

 new columns. These are the only statements seen in regard to the ar- 

 rangement of interambulacral plates in Palseechinoids, except the general 

 remark frequently met with, that the columns (ranges) of plates fail to 

 reach the poles. • - 



Before describing the plate arrangement in detail it may be well to 

 state the case in brief. Echinoderms grow by the addition of new plates 

 to the corona around the abactinal area and by the increase in size of 

 previously formed plates. As the new plates of each ambulacrum or in- 

 terambulacrum are formed they are inserted between previously formed 

 plates of the area and the genital or ocular plates (see plate 3, figure- 13) ; 

 therefore those plates on the lower part of the test were the earliest formed, 

 and passing dorsally we get progressively the later and later added plates, 

 built as the individual grew aborally. 



In Melonites multiporus, at the oral termination of the corona and in 

 immediate contact with the peristome, each ambulacrum has a rowf of 

 four plates and each interambulacrum a row of two plates X (plate 2, fig- 

 ures 2 and 3). Passing from this part progressively upward, or dorsally, 

 we find an increase in the number of columns in both the ambulacra and 

 interambulacra. In the ambulacra the two outer plates at the base, a, 6, 

 with additions dorsally, form the two lateral columns of the area, and 

 the two median plates at the base, a' b', with additions dorsally, form 

 the two median columns of the area (plate 2, figure 4, and plate 4, 

 figure 18). In the interambulacra new plates are added to form each 

 column, and also new columns are added with great regularity (plate 2, 

 figure 2; plate 3, figure 12 ; plate 4, figure 18). New columns are added 

 rapidly at first, and they attain their greatest number at a point of some- 

 what variable distance above the ambitus (plate 4, figure 18). 



Near the anal area the number of columns of interambulacra decreases 

 as well as at the oral area, but for a different reason, as discussed. In 

 the figures the several columns are numbered progressively, correspond- 

 ing to the number of columns attained by the individual, and therefore 



* See figure of Palceechinus gigas, plate 7, figure 38. 



fThe term row in this paper is limited in use to series of plates lying in one horizontal plane, in 

 contradistinction to the term column, which is applied to series of plates lying in a vertical plane. 



X The structure and development of the ambulacrum and the position and development of the 

 first three interambulacral columns, numbers 1, 2, 3, plate 2, figures 2, 3, etc., as described in this 

 paper, were worked out by R. T. Jackson. 



