216 MB. W. PEECY SLADEN ON THE 



be described is located in the median iuterbrachial line, and 

 consists o£ a number of greatly compressed spiuelets or lamellae 

 arranged in vertical parallel lines. Eacb of the lines thus 

 formed is equal in length to tlie height of the two series of 

 marginal plates and is invested with membrane. Ten or more 

 such lines or pseudo-lamellse are present on either side of the 

 median iuterbrachial suture ; and these do not stand quite per- 

 pendicular to the plane of the marginal plates, but are directed at 

 a slight angle towards the median suture. At the upper or 

 aboral estremity, where the organ terminates on the dorsal area, 

 there is a grouping of the spiuelets that belong to the dorsal 

 membrane, which are also rather more robust here than else- 

 where on the surface. At the lower extremity of the organ, the 

 outer lamellae are rather shorter than the inner ones, and each 

 being less than the next inward, a rounded outline is given to 

 tbe lower or adoral extremity of the organ. Five or six flattened 

 spiuelets, directed upward and slightly inward, are placed round 

 tbis semicircular margin and form an elegant fringe or comb, 

 "wbich closes over, as it vs^ere, upon the series of lamellae. 



On examining this organ microscopically it is found that the 

 lines or lamellae are made up of series of small lamellae, placed 

 end to end together, thus forming an apparently continuous 

 line. Each component part or individual lamella stands up- 

 right upon its own rounded scale-like base ; and the lamellar 

 plates are made up of a single series of delicate rods united by 

 irregular dissepiments, the whole structure being covered with a 

 membrane, which appears to have been furnished with vibratile 

 cilia. The scale-like plate which forms the basal portion is 

 directly superposed upon the surface of the marginal plates, the 

 parts occupied by the organ being slightly hollowed out for its 

 reception. The outermost lines (pseudo-lamellse) are composed 

 of thicker individual lamellae than any of the others, and these 

 integral lamellae stand wider apart and resemble flattened spine- 

 lets, each built up of several series of rods. On the upper 

 portion of each line transition can be traced from the delicate 

 lamellae, above described, to the simple rounded cylindrical 

 spiuelets of the dorsal membrane. 



Judging from the position and character of this organ, as well 

 as from its relation to the dorsal area, it is not improbable that 

 it functioned as a percolator ; and in such a case it might 

 perhaps be looked upon as the homologue of the minute ciliary 



