INTRODUCTION. XV 
ambulacral system exhibits a “retarded” development in Zurystroteria, and an 
“accelerated” one (leading usually to a quadriserial arrangement of the tube-feet) in 
Leptostroteria, yet the latter pass through a stage in which this crowding of the plates 
is absent; (8) the marginal plates are conspicuous throughout life in the Phanerozonia, 
though they become obscured in the course of development in the Cryptozonia. In 
each case, therefore, we can divide the subclass into two groups, of which one is more 
primitive than the other; and it is found that, with very few exceptions, the older group, 
as determined by one set of characters, corresponds with the older group as determined 
by the other two. Finally, owing to the importance of the ambital skeleton, and the 
comparative ease with which it can be observed, the names Phanerozonia and Cryptozonia 
are adopted for the two subdivisions of the Euasteroidea. It will be seen from this that 
Sladen, discarding artificial systems, to which he had a rooted objection, endeavoured 
to build his classification on a strictly morphological foundation. What success he 
achieved in this must be left for specialists, and above all for time, to determine; but 
at least it may be said that though some leading authorities are not wholly satisfied 
with his system, no decisively superior one has yet been introduced. It has never been 
accepted by Perrier, whose rival classification by pedicellarize has, however, found but 
little general favour; while it is satisfactory to note that Delage and Hérouard, whose 
competence to express an opinion will not be questioned, have recently adopted Sladen’s 
system in their ‘Traité de Zoologie Concréte’ (tome iii. “Les Echinodermes”: Paris, 1903). 
The text of the Report includes full descriptions of all the species collected, not only 
by the ‘Challenger,’ but by the other British deep-sea dredging-expeditions—the 
‘ Lightning,’ ‘ Porcupine,’ ‘ Knight Errant,’ and ‘ Triton.’ In addition to this a complete 
list is given of all the known species of recent Asteroidea (810 in number), together with 
their synonyms and their geographical and bathymetrical distribution ; and this list is 
especially valuable because, with very few exceptions, the recognition of the species 
included was based on Sladen’s personal observation. 
He has been charged in some quarters with a too great readiness to multiply species, 
and it may be admitted that, of the two groups into which systematists naturally fall, 
he was on the side of those who emphasize small differences. His views on the subject 
are, indeed, fully set out in his paper on the Echinodermata of the Korean Seas (4. p. 429), 
in the course of which he says: “However seriously the multiplication of frivolous 
species may embarrass a classification, the wholesale grouping, or, in other words, the 
unbounded extension, of the limits of specific character is productive of much more 
injurious results, in that it curtails the precision of definition and, while ignoring 
environment as a factor, divests nomenclature of one of its highest and most important 
qualities”; and he goes on to insist on the importance of recording the influence of 
habitat, and of attaching due weight to regional varieties. If, therefore, it has been found 
necessary to reduce some of his species to the rank of mere varieties, and to regard 
others as immature forms, that is no more than the natural consequence of increased 
knowledge, and is no disparagement to a work which has, as a whole, stood the test of 
time remarkably well. 
In the same year (1889) that saw the final publication of the ‘Challenger’ Report, 
