30 
TH. MORTENSEN, 
(Schwed. Südpolar-Exp. 
character, being exceedingly variable, as sufficiently shown by De Loriol. As a 
matter of fact I find this irregularity in the abactinal series of primary interambu- 
lacral tubercles more or less developed in all the specimens examined by me, not 
a single one having a quite regular series (viz. with a large tubercle on every plate, 
gradually diminishing in size towards the apical system). In several cases the pri- 
mary tubercle has even wholly disappeared on every second abactinal plate. * 
STUDER (Op. cit. 1880) points out a considerable difference in the structure of 
the pedicellariæ as a distinguishing feature between alternans and Dufresnii (Taf. I. 
Fig. 2, 3). De Loriol is inclined to see likewise a real difference herein. Studer’s 
figures give, however, the proof that these differences, though conspicuous enough, 
are without any value for the purpose of distinguishing alternans from Dufresnii , 
because one (Fig. 2) represents an ophicephalous, the other (Fig. 3) a tridentate 
pedicellaria. Real, constant differences in the pedicellariæ by which two species 
might be distinguished I have been unable to find. STUDER further states that the 
spines of alternans are more pointed than those of Dufresnii. I am unable to see 
any difference in the spines either; they certainly vary somewhat both in length and 
thickness, but these variations are too inconsiderable and inconstant to be used as 
specific characters. 
In the careful description of Dufresnii and alternans given by TROSCHEL (Op. cit.) 
only two more important differences are pointed out, viz. that of the tuberculation, 
which has been shown to be non-existent and the colour, alternans being stated to 
be brown, which would certainly be a very conspicuous difference from the eminently 
characteristic green colour of Dufresnii. It is quite true that the colour of the type 
specimens of alternans was brown, but this depended on their being insufficiently 
* In the »Hassler» Echini (p. 7) Agassiz speaks of »the property possessed by the Arbaciadæ of re- 
sorbing at any time during their growth the primary abactinal interambulacral tubercles, and changing them 
into a sort of chagrin, or finely granular, nearly bare abactinal star». — Such a resorption of tubercles 
(with the spine, muscles etc.) and the transformation of them into epistroma (which seems to me to be the 
meaning of the sentence quoted) is highly improbable and — ■ as far as I know — quite without any proof. 
It is further quite unnecessary for understanding the alternation of the tubercles in this and other species 
of Echini. The fact is that no tubercle and spine is ever formed on the plates lacking them in grown 
specimens. This may be easily confirmed on examining somewhat carefully the upper plates in young speci- 
mens. Whereas thus the alleged property of resorbing the tubercles and transforming them into epistroma 
appears very improbable and without any supporting facts and observations, the inverse case, viz. the re- 
sorption of the epistroma and its use partly in the formation of the tubercles, seems put beyond doubt 
through the beautiful researches of Loven on the changes undergone by the test of the young Arbacia 
lixula ( aquituberculata ) (Echinoidea descr. by Linnaeus, p. 86 — 94). 
I may recall here my note (»Ingolf» Echinoidea II. p. 41) on the alleged resorption of injured spines 
in Urechinus described by Agassiz (Panamic Deep Sea Echini p. 153, 159 — 60). That any resorption has 
taken place in these cases is highly improbable, and no facts tend to show it. Most probably the cases 
described by Agassiz are only the results of the rough treatment of the specimens in the dredge. It may 
also, to more or less extent, be due to autotomy. Though hitherto not known in the Echini autotomy has 
now been proved to occur also here through the beautiful researches of O. Poso. (Ricerche biologiche ed 
istogenetiche sugli Echini regolari. Archivio Zoologico. II. 1909. p. 463). 
