PROCEEDINGS OF THE SOCIETY. 1005 



Prof. Bell read his paper " Notes on the Structural Characters of 

 the Spines of the Echinoidea — Cidaridae " (see p. 846). 



Dr. Carpenter, C.B., said he must confess that he did not at 

 present clearly understand Prof. Bell's meaning as conveyed in this 

 paper. The spines of the ordinary Echini, whenever they were not 

 annual spines — which were shed and renewed every year — presented 

 certain well recognized features. In the largo spines of the many 

 tropical forms, they had in the transverse sections a series of well- 

 marked rings of growth resembling the annual rings of the trunk of 

 a tree, and these might go on increasing indefinitely because every 

 new growth was added in the same way. But in the Cidaridce there 

 was nothing of the kind ; there the whole interior of a spine seemed 

 to be formed continuously, so that the cylindrical interior contained 

 passages prolonged from the internal passage or solid network which was 

 occupied by a protoplasmic substance. Now, when that kind of sheath 

 was first formed he did not deny that it might increase up to a certain 

 point, but he had yet to be informed that the interior continued to 

 undergo an increase after the external portion had become hardened. 

 They knew perfectly well now how lines were enlarged, and that 

 there was a process going on there of continual removal of old matter 

 and the addition of new. The old notion of interstitial swelling out 

 was now given up, and he could not think of it as going on in the 

 case of a spine. The only other mode of increase would be by the 

 removal of the interior portion of the sheath and by addition to the 

 external portion. So far as his observations went he could see no 

 evidence that when the cylinder had once been encased by this nearly 

 solid sheath, the internal portion of the spine underwent any in- 

 crease. 



Prof. Stewart said that so far as regarded the spines of the 

 Cidaridce his observations entirely tended to show that having once 

 become invested with this calcareous sheath, the vitality of the spine 

 was so lowered thereby that it allowed of the accumulation of para- 

 sites of all descriptions which infested these spines and these only. 

 The process of investment amounted in fact to an arrest of growth. 

 By means of a drawing on the black-board lie showed that these 

 spines were increased by additional layers being added to them, after 

 which they became encrusted and SerpulcB, &c., became attached to 

 them. In the Goniocidaridce there was less liability to this. 



Dr. Carpenter said that his experiences on these points entirely 

 coincided with those of Prof. Stewart. 



Prof. Stewart further said that wherever the spines were found 

 in frictionul contact with each other the interstices would be seen to 

 be filled up with calcareous matter, just as similarly occurred when 

 two portions of bone were in contact in cases of osteo-sclerosis. 



I'rof. Boll said he was sure that the meeting would understand 

 that ho had made himself acquainted with what had been done ou 

 these subjects by Dr. Carpenter and Prof. Stewart, but what troubled 

 him to understand was how if the adult spine had a certain amount 

 of incrustation, the crust should be proportionately thinner in the 

 larger tlian in the smaller specimens. Ho had carefully examined 



