ZOOLOaY AND BOTANY, MICROSCOPY, ETC, 253 



Echinoidea and some Holotliurians. Found as cells containing 

 yellowisli-green spherules in the blood-vessel of the intestine and 

 in the ambulacral saccules, they vary greatly in size. When they 

 unite into larger masses, the addition of acetic acid reveals the 

 presence of a surrounding layer of non - nucleated protoplasm. 

 Larger brown spherules are also found intermixed with these, and 

 they become more and more deeply pigmented and get to resemble 

 the coloured corpuscles, taking on as they do the coarser granulation 

 and the irregular form. In fine, there is every phase of development 

 between the greenish-yellow and the brown corpuscles ; so that we 

 have here an exception to the general rule, that in the Metazoa every 

 cell arises from the transverse division of another cell. 



As to the function of these bodies, there is some reason for 

 suggesting that it is respiratory ; the colouring matter, which is 

 instable, is highly ferruginous ; this instability is very remarkable in 

 Sjpatangus purpureus. Purple, blue, green, olive, or yellow cor- 

 puscles may be found in the blood-vessels ; some of these are, at any 

 rate, nothing but altered brown corpuscles, and there can be no doubt 

 but that the purple colour of the Spatangus is due to a decomposition 

 of the brown matter. 



Contrary to the opinion of Hoffmann, Mr. Geddes thinks that 

 spermatozoa never escape naturally into the perivisceral cavity. The 

 infusorian parasites which are so abundant in the cavity of Strongylo- 

 centrotus lividus would appear to reach it by some accidental lesion of 

 the walls of the intestine. 



The irregular bodies sometimes found in the ambulacral saccules 

 are shown to be nothing more than fragments of muscular fibres ; it 

 is incorrect to imagine, as Williams and others have, that the peri- 

 visceral fluid, the ambulacral fluid, and the blood contain the same 

 histological elements ; the three systems are really independent. 



Pedicellarise and Muscles of Sea-TJrcliin.* — Messrs. P. Geddes 

 and F. E. Beddard have recently studied the pedicellarise, which have 

 of late received fresh attention from several good observers. They 

 find that in the snake-headed form of pedicellaria the muscles uniting 

 the head to the stem are not attached directly to the calcareous parts ; 

 most of them terminate in a series of loops outside the latter. Two 

 fasciculi are continued beyond the rest, mingle with the semicircular 

 muscles of the valves, and end freely in a tuft of meshes in the 

 middle of the triangular valve-muscle. A peculiar reticulated struc- 

 ture, not acted upon by dilute acetic acid, and lying in the intervals of 

 the muscular fibres just described, is probably elastic and acts as 

 a ligament, opening the valve when the adductor muscles are not 

 contracted. In the tridactyle and gemmiform pedicellarise these liga- 

 mentary structures are very delicate, and the muscles of the outer 

 end of the stem are inserted on the calcareous parts instead of forming 

 loops. 



Outside each valve of the gemmiform type lies a gland covered 

 by two layers of muscular fibres and a layer of cylindrical epithelium. 



* Comptes Eendus, xcii. (1881) pp. 308-10. 

 Ser. 2.— Vol. I. T 



