36 



W. PERCY SLADEN. 



of the rapidly growing adjacent basal plate. Even when the 

 Asterid has attained maturity the position of the water-pore 

 / (i. e., the madreporite) is more or less lateral in its position on 



the plate, a fact which has been observed by Ludwig^ who also 

 remarks on the similar position of the water-pore in relation to 

 the punctured plate both in Ophiurids and Crinoids. 



These points alone are sufficient to indicate that the relation 

 of the water-pore to the plate which may be associated with it 

 is not of such an intimate nature as has generally been 

 supposed ; and it is still more conclusive to find that in a large 

 number of Starfishes the madreporite, even in the adult form, 

 is situated external to, and often independent and well separated 

 from, the basal plate. Without enumerating cases in detail it 

 will suffice to mention species of Peutaceros, Astrogonium, 

 Tosia, Pentagonaster, Stellaster, Goniodiscus, Fer- 

 fC dina (?) ; together with Gymnasteria cariuifera, Der- 

 ;,.■ masterias imbricata (fide Gray), Scytaster variolatus 

 (? part), Ophidiaster pyramidatus (fide Viguier), and 

 Zoroaster fulgens. The total disconnection of the madre- 

 porite from the oral plates in the Astrophytidae (Astrophy- 

 ton, Tr ich aster, &c.) may likewise be referred to as supporting 

 the same view. 



These circumstances unquestionably prove that the position 

 of the water-pore is not in a definite and unchangeable relation 

 to a given plate and that its presence per se is not a reliable 

 index of the homology of that plate. Furthermore, they 

 strengthen the view that morphologically the position of the 

 water-pore is a travelling factor and may occupy any position 

 upon the interradial line — an argument that derives further 

 support from what is known of the embryology of the group. 



It may also be mentioned incidentally here that a similar 

 travelling position along the interradial line has already been 

 shown by A. Agassiz^ to occur in the phylogenetic history of 

 the periproctal aperture in the Echinoids. 



Trusting upon the supposed homology of the plates punc- 



' ' Entwickl. Ast. gibb.,' p. 160. 



2 'Bull. Mus. Comp. Zool.,' 1S69, ser. 3, No. 9, p. 295. 



