SHELL. 



569 



which rises up in ridges upon the exterior. 

 Hence it would appear that, like endogenous 

 trees, whatever additions these spines may 

 receive in length, they can receive little or 

 none in diameter. The slender, almost fila- 

 mentary species of the Spatangacece, and the 

 innumerable minute hair-like processes at- 

 tached to the shell of the Clypeasteridce, are 

 composed of a like regular reticulated tissue ; 

 many of these are extremely beautiful objects 

 when examined with the microscope without 

 any preparation. It is interesting also to 

 remark, that the same structure presents itself 

 in the PedicellaricE, which are found upon 

 the surface of many Echinida, and which have 

 been so great a source of perplexity to 

 naturalists. The complete conformity which 

 exists between the structure of their skeleton, 

 and that of the animal to which they are 

 attached, would seem to remove all reasonable 

 doubt that they are truly appendages to it ; 

 as their actions also would indicate. 



The same structure presents itself in the 

 calcareous plates which form the less perfect 

 skeletons of the Asteriadce, and also in their 

 spines, when these (as in the large Goniaster 

 equestris) are furnished with a calcareous 

 frame-work, and are not mere projections 

 of the hard integument. It is also met with 

 in the family Ophiuridce, which forms, in 

 some respects, the transition to the Crinoidal 

 group ; but the calcareous skeleton is here 

 generally subordinate to the firm and almost 

 horny integument. In the Cnnoidea, on the 

 other hand, the calcareous skeleton is highly 

 developed, and its structure is extremely 

 characteristic. This is well displayed in the 

 recent Pentacrinus Caput Medusce, the stem 

 and branches of which are made up of a 

 calcareous net-work, closely resembling that 

 of the shell of the Echinus. There is ex- 

 hibited, moreover, in a transverse section 

 of the stem of Pentacrinus, as in the spines of 

 Echinus, a certain regular pattern, which 

 results from the varying dimensions of the 

 areolae in different parts. This pattern, 

 formed by the extension of five pairs of rays 

 (strongly reminding us of the medullary rays 

 of plants) from the centre towards the cir- 

 cumference, is frequently well preserved in 

 the fossilized stems of Pentacrini, and varies 

 in different species sufficiently to serve as 

 a distinctive character. In the round-stemmed 

 Encrinites, a transverse section of the joints 

 exhibits a simple concentric arrangement. 



It only remains for us to notice the order 

 Ilohthurida:, in which, as is well known, the 

 calcareous skeleton of the other Echinoder- 

 mata is reduced to its most rudimentary con- 

 dition ; never forming a complete and con- 

 nected framework, but only showing itself in 

 detached pieces, the disposition of which 

 is extremely variable. In the typical Holo- 

 t/inria, there are five solid calcareous plates 

 around the mouth, in which the calcareous 

 reticulation is very characteristically seen. 

 Each of the tentacula, also, has a small cal- 

 careous disk at its extremity, which presents 

 a sort of rude sketch of the beautiful struc- 



ture of the rosette that supports the ambu- 

 lacral suckers of the Echinus. 



There can be no reasonable doubt that this 

 peculiar arrangement is universal throughout 

 the group, since it has been detected in cha- 

 racteristic examples of every one of its prin- 

 cipal subdivisions. And, consequently, as no 

 similar calcareous reticulation is found in the 

 internal or external skeleton of any other 

 animal, even the minutest fragment which 

 distinctly presents this structure may be re- 

 ferred with certainty to an Echinoderm. 

 And this structure is perfectly preserved, even 

 after the substance has been infiltrated with 

 calcareous matter in the act of fossilization, 

 and has become so completely mineralised, 

 that the disposition to rhomboidal fracture 

 makes it difficult to obtain a section in any 

 other direction than that of the plane of 

 cleavage. As already remarked, the elemen- 

 tary structure is essentially the same every- 

 where ; so that it might not be possible to 

 determine from a very minute fragment whe- 

 ther it formed part of the shell of an Echinus, 

 Cidaris, or Spatangus, a portion of the frame- 

 work of an Asterias, Op/iiura, or Hololhnria, 

 or entered into the composition of the stem 

 of an Encrinlte. But where any regular pat- 

 tern is displayed, this is frequently sufficient 

 to distinguish the genus, or even the species, 

 to which the fragment belonged. This is 

 certainly the case in regard to the spines of 

 Cidarites and the stems of Pentacrinitcs ; and 

 will probably be found no less true in other 

 instances, when these beautiful structures 

 shall have been more extensively investigated. 



Crustacea. The structure of the shell in 

 Crustacea has been hitherto examined only in 

 the Decapod order ; and that of the common 

 crab (Platycardnus pagurus) alone has been 

 subjected to a minute investigation. It is in 

 the Decapod order that the shell attains its 

 most perfect development, and contains the 

 largest proportion of mineral matter : the 

 special respiratory apparatus in this order 

 being so elaborate as to render unnecessary 

 any participation of the general tegumentary 

 surface in the function of respiration. (See 

 vol. i. p. 752.) 



The shell of the Decapod Crustacea con- 

 sists of three layers; namely, 1. a horny 

 epidermic membrane covering the exterior ; 

 2. a cellular or pigmentary stratum ; and 3. a 

 calcareous or tubular substance. The horny 

 epidermic membrane is easily detached from 

 the subjacent layers, after the shell has been 

 immersed for a time in dilute acid ; it is thin 

 but tenacious, presenting no trace of structure, 

 though it may exhibit markings on the under 

 surface, derived from its contact with the cel- 

 lular layer beneath. The pigmentary stratum 

 is very thin in the crab and lobster ; but in 

 some other Decapods it is much thicker. In 

 Sri/llurus latus, it is stated by M. Lavalle to be 

 the thickest of the three layers of the shell ; 

 and in the cray-fish and many other species, 

 according to the same observer, it seems made 

 up of a considerable number of layers, its ver- 

 tical section being traversed by several ex- 



