BIHANG TILL K. SV. VET.-AKAD. HANUL. BAND 13. AFD. IV. N:0 5. 87 



on the vippcr side, and cousists of simply globular knobs spa-r 

 ringly clustered in the narrow interstices. At the ambitus 

 these knobs increase in number and size, encircle the bases 

 of the tubercles and attain their highest developmcnt: on the 

 ambulacra near the middle sutnre and on the interradia out- 

 side the primordial tubercles, but diminish on the inward 

 portion of the disk, and Avliile there the formation of tubercles 

 is arrested, they are bedded more or less suddenly in a conti- 

 nuous, corapaet layer or crust, even or slightly swelling, con-i- 

 taining the protuberances reduced into minute glossy knobs, 

 nearly sunk and disposed in linear but slight rugosities divari- 

 cating towards the sutures and more or less distinctly corres- 

 ponding from plate to plate, Tab. 7, jig. 3, 4. 



Such is in its general features the epistroma of the adult 

 Arbacia. With a view to make it somewhat better under- 

 stood, and though at the peril of wandering widely from my 

 proper subject, I shall venture here to subjoin some observa- 

 tions on the earlicr phases of its appearance. On Tab. 8 a 

 magnified representation is given, in Jig. 1 of the calycinal 

 system and parts of the ambulacrum III and the interradium 

 3, and in jig. :? of a portion of tho ambulacrum I, both taken 

 from the test of a young specimen of the Arbacia Kquituber- 

 culata Bly., 5 mm. in diameter, from which the soft tegu- 

 ments had been carefully removed. 



As far as it may be allowable to conclude from observa- 

 tions on one or two species, it seems that the epistroma, ori- 

 ginally to all appearance a modification of the membranous 

 envelope of the pluteus, is a thin cuticular layer external to 

 the matrix layer of the spine-bearing tubercles, and to the un- 

 derlying skeletal plates, distinct from both and distending as 

 they grow. It calcifies after its owu manner, along certain 

 regular lines formino- toerether a svstcm of ridoes which tend 

 to unite into a network of triano-nlar meshes havino- its centre 

 within the calyx and radiating from there över ambulacra 

 and interradia. At that early stage, when the Echinid has 

 used up the nutriment derived from the pluteal state and, 

 with the alimentary canal opened and functioning, has begun 

 to feed, the growth of the perisome of the young Arbacia by 

 means of new plates added at the aboral margins of those al- 

 ready formed, ambulacral and interradial, is so rapid, and the 

 råte of formation of spines and tubercles relatively so slow, 



