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see therein a difficulty for the derivation of the Echinothurids from 
the Lepidocentrids. Somewhere in the course of phylogenetic devel- 
opment the external branchiæ must have come into existence, and 
their rather rudimentary condition in the Echinothurids is decidedly 
in favour of the suggestion that they have arisen here or in their 
direct ancestors. — And then there is another fact which is at 
least an important argument for regarding the Echinothurids as 
the group of the Diadematoidea nearest related to the Palæozoic 
forms, even .if it cannot perhaps directly prove the genetic con- 
nection between the Echinothurids and the Lepidocentrids, viz. the 
existence in the former of well developed Stewarts organs. To 
place the Echinothurids at the top of the Aulodonta, as does Jack- 
Son, is certainly not in accordance with this important anatomical 
feature, the other Diadematcids having no Stewarts organs. (Only 
in Echinothrix quite rudimentary Stewarts organs are found, ac- - 
cording to A. Agassiz & H. L. Clark. Op. cit. p. 142. Pl 60.4). 
Ålso the existence of many ambulacral plates on the peristome is, 
in my opinion, doubtless a primitive character. The suggestion of 
Dåderlein (Echinoidea d. deutsch. Tiefsee-Exp. p. 82) that this 
may perhaps be a newly acquired character, seems to me without 
any real support. (It is only fair to state that Dåderlein does 
not maintain this as his own opinion, only as a possibility). i 
I think it has been shown herewith that the existence of gills 
im the Echinothurids is an even less valid argument than the multi- 
columnar interambulacra of Lepidocentrids, against the genetic con- 
nection between these two families. In the masticatory apparatus 
Jackson has shown some differences to exist, viz. the upper sur- 
faces of the half pyramids being pitted in Echinothurids (as in all 
Diadematoids), while they are smooth in the palæozoic forms; styloid 
Processes are distinet in the Echinothurids, not found in me 
reservoirs of the fluid within the peripharyngeal sinus by changes of 
pressure due to the econtraction of certain muscles. 
15% 
