568 



SHELL. 



structure in the calcareous rosette, with 

 which, as long since observed by Monro, 

 the sucker at the extremity of each am- 

 bulacral tube is furnished. But it is in the 

 spines with which the shell is beset, that the 

 most remarkable displays of it are to be met 

 with ; for it is there disposed in connection 

 with solid ribs or pillars, which increase the 

 strength of these organs, in such a manner as 

 to constitute a most regular and elaborate 

 pattern, which appears to differ in every 

 distinct species. When we make a thin 

 transverse section of almost any spine be- 

 longing to the genus Echinus, we are at once 

 made aware of the existence of a number 

 of concentric layers, arranged in a manner 

 that strongly reminds us of the layers of 

 wood in the stem of an exogenous tree. 

 The number of these layers is extremely 

 variable ; depending, not merely upon the 

 age of the spine, but upon the part of its 

 length from which the section is taken. The 

 centre of the spine {fig. 424. a.) is filled up 



Fie. 424. 



Transverse section of spine of Echinus : a, medullary 

 centre ; bb, first layer of solid pillars; cc, dd, ee, 

 ff, successive rings of growth. Magnified 45 

 diameters. 



with the same kind of calcareous net-work as 

 that of which the shell is composed ; and 

 this is sometimes so delicate, as to appear 

 as if made up by the interlacement of mere 

 threads. This medullary centre is bounded 

 by a row, more or less circular according to 

 the form of the spine (which is sometimes 

 angular), of open spots (b, b, b), in which it is 

 deficient : these, on a cursory inspection, 

 might be supposed, from their transparency, 

 to be void spaces ; but a closer inspection 

 makes it evident that they are the sections 

 of a circular row of solid ribs or pillars, 

 which form the exterior of every layer. 

 Their solidity becomes very obvious when we 

 either examine a section of a spine whose 

 substance is pervaded (as frequently happens) 

 with a deep colour, or when we look at a 

 thin section of any spine by polarised light. 

 Around the first circle of these solid pillars, 



we find another layer of the fibro-calcareous 

 net-work, which again is bounded by ano- 

 ther circle of solid pillars, whose transverse 

 sections are seen at c, c, c. The same ar- 

 rangement may be repeated many times, (eld, 

 ee). On looking at the outer border of the 

 section, we observe that the rounded sides 

 of these pillars (/,/) form a series of pro- 

 jections with hollows between them ; and 

 these exactly correspond with the projecting 

 ribs and furrows which we may notice run- 

 ning along the natural surface of the spine 

 when we examine this with a magnifying 

 glass, or even (in some instances) with the 

 naked eye. 



Although there is nothing like interstitial 



D ~ 



growth in the shell or spines of the Echinus, 

 yet both are progressively enlarged by the 

 addition of new matter. The polygonal 

 plates of which the shell is composed are 

 separated from each other by a membrane 

 that passes into every suture ; and the 

 margins of each plate appear to receive 

 periodical additions, by calcareous deposit 

 in the substance of this membrane. In this 

 manner the globular form of the entire shell 

 is preserved, whilst it undergoes progressive 

 enlargement ; new plates being added, as 

 they may be required, round the anal orifice 

 of the shell (Agassiz). There can be little 

 doubt that the spines are, in like manner, 

 periodically augmented in diameter by suc- 

 cessive formations or acts of growth, which 

 take place in the investing membrane ; and 

 a longitudinal section of the spine makes it 

 evident that these additions not only sur- 

 round the preceding deposits from the base 

 upwards, but pass considerably beyond them, 

 thus adding to the length of the spine. The 

 consequence is, that a transverse section 

 taken near the base of the spine will exhibit 

 all the layers of which it is made up, each 

 layer being narrow, and the central medulla 

 small. A section taken at about the middle 

 of the length may very probably not cut 

 across the original spine nor the older layers, 

 which do not reach so far ; and a section 

 taken across the spine near its apex will 

 only traverse the one or two layers last 

 formed. Nevertheless, in many species, the 

 spine is larger at that part than near its base ; 

 but the large size is due to the great ex- 

 pansion of the medullary centre, which is 

 composed of a very loose calcareous reti- 

 culation. 



The structure of the shell of the Echinus 

 is repeated in that of the three genera which 

 may be regarded as the types of the principal 

 subdivisions of the order Echinida, namely, 

 Cidaris, Clypeaster, and Spatangus : there can 

 be no reasonable doubt, therefore, that it is 

 universal throughout the group. The spines, 

 however, of Cidaris, present a marked vari- 

 ation from the plan of structure exhibited in 

 Echinus ; for they are usually nearly cylin- 

 drical in form, destitute of concentric layers, 

 and composed of a calcareous reticulation en- 

 veloped in a cylinder of a solid, apparently 

 homogeneous substance, chiefly calcareous, 



