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XXII. On certain Organs of the Cidaridce. By Charles Stewart, Esq., F.L.S., 

 M.B.C.S., Lecturer on Comp. Anat. St. Thomas's Hospital. 



(Plate LXX.) 



Bead December 6th, 1877. 



oOME years since, whilst examining a spirit-preserved specimen of Dorocidaris papil- 

 lata, I first noticed the organs which it is the object of this paper to describe. An 

 undetected leak in the cell in which they were preserved unfortunately allowed them to 

 dry before a drawing had been made. To the kindness of the Rev. A. M. Norman I am 

 indebted for two specimens, which have enabled me to confirm my observation, and 

 make the necessary drawings. 



Of the regular Echinoidea the families Diadematidae, Echinometradae, and Echinidae 

 have long been known to possess external branchiae in the form of five pairs of conical 

 hollow processes of the outer margin of the buccal membrane. The branchiae project 

 outwards into the surrounding water, their interior being bathed by the fluid of the body- 

 cavity, of which chamber they are diverticula. Their under surface is amplified by 

 secondary diverticula, the whole organ having its walls strengthened by small calcareous 

 plates, and sometimes spicules. Large cilia clothing its exterior, keep a constant current 

 of water over it. The bases of the branchiae are attached to the edges of the ten notches 

 which are usually present at the buccal margin of the corona. As might be expected, 

 these notches are most marked where the branchiae are largest, e. g. Toxopneustes, 

 Splicer echinus, &c. 



The existence of such branchiae in the Cidaridae was denied by Muller ; but in the 

 magnificent ' Revision of the Echini ' of Prof. A. Agassiz, p. 694, he says : — " In the 

 Cidaridae these gills are found in cuts of the buccal membrane itself, close to the acti- 

 nostome (Muller denies their existence ; yet he has himself figured the cuts through 

 which the gills pass in a species of Cidaris)." 



I have carefully removed the spines one by one from the buccal membrane of Doroci- 

 daris papillata preserved in spirit, and then examined it under the microscope without 

 finding a trace of cuts or branchiae (PL LXX. fig. 1); but in dry specimens the actinostome 

 appears notched from the shrinking of the membrane, which here occupies a gap left 

 between the interambulacral plates; this membrane shows a few creases, but nothing of 

 a branchial character. 



If the abactinal or dorsal half of the corona be now removed, five remarkable organs 

 will be seen (fig. 2), which I believe must functionally take the place of the absent 

 external branchiae. They are situated immediately beneath the outer forked extremities 

 of the five compasses, and are diverticula of the chamber that occupies the intervals 

 between the jaws and their muscles, and which is bounded by the reflection of the inner 

 lining (peritoneum) of the test over them. 



