H. L. Haiokins — Apical System in the Holectypoida. 9 



of my researches in a scheme for the classification and evolution of 

 the group in a paper already partly prepared. 



B. The points to be examined. 



Tlie apical cycles of plates are of fundamental importance in an 

 Echinoid. IS^ot only are they directly associated with the reproductive 

 system, the water- vascular system, and some of the more delicate of 

 the sensory organs of the animal, but it is from their outer margins 

 that all the new coronal plates arise. The differences of structure 

 Avhich are shown in the apical system in various groups must therefore 

 be indications of essential differences in the structure and development 

 of both the skeletal and softer tissues of the body. From the fore- 

 going sentences it will be realized that I differ considerably from the 

 opinions expressed by Duncan (1889), and tend to regard the apical 

 system as a phylogenetic index of the first importance. Nevertheless, 

 I do not absolutely follow Pomel (1883) in treating the structure as 

 the chief OY only guide in classification. 



Many careful studies of the apical system of Echinoids have been 

 made, and the contrasts in its structure figure largely in the diagnoses 

 of the larger and smaller groups. Many of the investigations were 

 undertaken as a result of the brilliant, though apparently mis- 

 taken, attempt of P. H. Carpenter to homologize the, apical cycles of 

 Echinoids with the calycal cycles of Crinoids. The Etudes of Loven 

 (1875) contain perhaps the most important contributions to a com- 

 parative study of the structure from this standpoint that have been 

 comprised in a single work. 



Within the limits of one group of Echinoids there are usually not 

 suflS.ciently marked differences of structure to render worth while the 

 publication of detailed descriptions of the apical system alone in 

 allied genera and species. In the case of the Holectypoida, however, 

 the changes in detail are so many and so striking that a comparison, 

 between genera at least, seems to lead to important results. No more 

 convincing proof of the intermediate position occupied by the group 

 between the various Echinoid orders can be supplied than that which 

 a study of the apical system affords. And conversely, it is because 

 the Holectypoida are an annectant group that the various forms show 

 such a plasticity of structural development. 



Almost the only reliable distinction between the early Holectypoida 

 and the primitive Liademoids lies in the position of the periproct. 

 However excentric the anus may be, it is always enclosed within the 

 boundaries of the apical system in the Regular Echinoids. In 

 Fygaster, for the first time in Echinoid history (if we except the 

 aberrant and somewhat problematical 'Acyclic' Silurian forms), the 

 periproct passes backwards to lie outside the cycles of the genital 

 and ocular plates. Thanks largely to the work of A. Agassiz and 

 S. Loven, we now know that many, probably all, of the Irregulares, 

 even of the Spatangoids, pass through a 'Regular' (though often 

 functionally aproctous) stage in the course of their ontogeny. This 

 means that the primitive apical system must, in every individual, be 

 ruptured for the retrogression of the anus. Another result is that, 

 even if the system regains its original quinqueradial symmetry, it 



