16 SALEXIA YAEISPIXA. 
it from a morphological point of view when compared to the single large anal 
plate of young Echini. Yet, while it may perhaps not be an embryonic fea- 
ture of the Echini which runs back through the Echinoid series of the earlier 
palaeozoic times, it may yet be one of those cases of the sudden reappearance 
of an ancient structual feature after a long period of time, as is also perhaps 
the five-valved actinal opening of Palceostoma. I am inclined to look upon 
the suranal plate of Salenia as strictly homologous with the central plate of 
the dorsal surface of Starfishes, which, while it recalls the Crinoidal affinities 
of the Echini, yet has not played the important part in the development of 
the Echinoid series which it did in the Starfishes or Crinoids. 
As far as the plates of the anal system are concerned, the embryonic type 
of many plates arranged in more or less irregular concentric rows round the 
anal opening, such as we find it in the oldest palaeozoic Echini, has remained 
remarkably persistent throughout the group to the present day. Even in 
the Spatangoids and Clypeastroids, in which the anal system has become 
disconnected from the apical system, the sanae general embryonic type has 
been retained in nearly all the principal groups, with the exception of a few 
types, such as Echinocidaris, and some of the Clypeastroids and Spatangoids, 
in which the number of anal plates has become reduced, and they form 
a pyramid over the anal system ; a structural feature, however, which we 
should remember is already found in some of the earliest known Crinoids. 
From what 1 have shown of the mode of breaking up of the anal system* in 
some very large specimens of Arbacia, tlie pyramidal anal covering may have 
been the eai'liest type of plates of the anal system in Crinoids, and the split- 
ting up of the plates of the anal system, at first few in number, may be a 
feature characteristic of the more recent Echini. Such a splitting up actu- 
ally takes place in the growth of the plates of the anal system of Salenia. 
In very young stages of S. PaUersoiii we have a suranal plate and five anal 
plates covering the anal system ; while in old specimens the anal system is 
covered by a number of smaller plates, due to the splitting up in part of the 
original plates at their apex, and in part to the formation of new plates 
round the anal opening with the increase in size of the anal system. 
How far we are justified in considering the anal plates of the recent Des- 
mosticha as homologous with the dorso-central plate of young Starfishes and 
of Comatula, seems somewhat doubtful. P. H. Carpenter! lias well discussed 
some of the difficulties to be met in adopting the view Loven and myself have 
* Challenger Echini, p. 50. t Quart. Journ. Mic. Soc, XVIII. 357. 
