227 
see therein a difficulty for the derivation of the Echinothurids from 
Lepldocentlids - Somewhere in the course of phylogenetic devel- 
opment the external branchim must have come into existence, and 
eir rather rudimentary condition in the Echinothurids is decidedly 
m favour of the suggestion that they have arisen here or in their 
dileet ancestors. — And then there is another faet which is at 
east an important argument for regarding the Echinothurids as 
the group of the Diadematoidea nearest related to the Palæozoic 
forms, even if it cannot perhaps direetly prove the genetic con- 
nection between the Echinothurids and the Lepidocentrids, viz. the 
existence in the formér of well developed Stewarts organs To 
place the Echinothurids at the top of the Aulodonta, as does Jack- 
son, ,s certamly not in accordance with this important anatomical 
ea ure, the other Diadematoids having no Stewarts organs. (Only 
in Echmothrix quite rudimentary Stewarts organs are found, ac- 
cording to A. Agassiz & H. L. Clark. Op. cit. p. 142. PI. 60 . 4 ). 
Also the existence of many ambulacral plates on the peristome is, 
in mi opinion, doubtless a primitive character. The suggestion of 
Doderlein (Echinoidea d. deutsch. Tiefsoe-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 D 6 der le in does 
not mamtain this as his own opinion, only as a possibility). 
I think it has been shown herewith that the existence of gilis 
in 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 
lOT cesses are disti net in the Echinothurids, not found in Lepido- 
which has 110 contact whatever with the external surroundin<»-s act 
(0 P mS Tf"/ AS S6t f ° rth m ° St eXCe,leDtly ^ B»‘her 
(Up. cit. p. 252) the function of the Stewarts organs is to act as 
reservoirs of the fluid within the peripharyngeal sinus by changes of 
pressure due to the contraction of certain muscles. 
15 * 
