262 H. L. Hawkins— Structure of Plnjllodes 



the phyllocles are almost flush with the level of the adoral surface. 

 The ambulacra, below the sub-petaloid tract, are composed of high 

 primaries until the expansion for the phyllode commences, when plate- 

 crushing rapidly develops, and the average height of the plates is 

 reduced by more than one-half. Plates of series a are the first to 

 suffer, and they become separated almost at once from the interradial 

 margin. Plate c is immediately afterwards crushed to a demiplate, 

 and the perradial portion of plate b is constricted, as in Pygwrm. 

 The crushing is accelerated to such a degree that the ' Pygurus- 

 phase ' of the structure persists for not more than two of the triple 

 sets of plates. Then plate a becomes notably high, and extends 

 laterally less than half the distance to the interradial margin, while 

 not only plate c, but also b, become reduced to the condition of 

 demiplates. The resulting structure is more apparent from the 

 figure than it could be rendered by description. The phyllode appears 

 as if it were made of two distinct ambulacra, one within the other; 

 the inner of more or less hexagonal plates, and the outer of twice as 

 many long narrow plates. Here and there a slight irregularity in the 

 degree of crushing gives the clue to the origin of this peculiar 

 structure. At about the middle of the phyllode, one of the central 

 series of a plates ou each side reaches out towards the interradial 

 margin in every example that I have examined. The triserial 

 arrangement of the pore-pairs is hardly appreciable, as the pores of 

 plate a are so far distant from those of plates b and c, which are 

 closely approximated. Moreover, the pores of the separate series are 

 not altogether similar in character. The marginal series of b and e 

 are paired, and appear as minute circles, but the a series of perradiad 

 pores are heterogeneous. The upper member of each pore-pair is 

 unusually large, while the lower pore is excessively minute, and often 

 quite invisible. It is in this case that the suggestion seems feasible 

 that the outer pores are for the passage of normal podia, and the inner 

 series for sensory tentacles. 



The value of the phyllode in classification. — I have already indicated 

 (Hawkins, 1910) that the ambulacral structures have as great a value 

 in the classification of the Holectypoida as they have among Regular 

 Echinoids. To a large extent the reliability of this feature in the 

 cases mentioned may be ascribed to the fact that it is not one of vital 

 importance to the individual. A multiplication of pore-pah"s, similar 

 to that found throughout the greater part of the ambulacra of both 

 groups, could be attained without any interference with the primary 

 character of the ambulacral plates. Por this reason, the presence of 

 so persistent a series of peculiarities as those found in the ambulacra 

 of the various groups of Itegulares and in the Holectypoida affords 

 a useful criterion of affinity. In the case of the phyllode, the feature 

 of adaptation to surroundings and manner of life is introduced. The 

 urgency of some such provision in the case of edentate forms without 

 labra has been emphasized at the beginning of the paper. But in this 

 case it would seem that the ethological considerations cannot eclipse 

 the fundamental phylogenetic evidence of the structure. One may 

 regard the labiate mouth of a Spatangid and the flo.scellate mouth 

 of a Cassidulid as being both adaptations to the same necessity. 





