742 DR. AV. B. CAEPEXTER ON THE STRUCTURE, PHYSIOLOGY, AND 



of parallel rods which 1 have already described, and by their being perforated for the 

 lodgment of a sarcodic axis." The extended researches which I formerly made on the 

 calcareous skeleton of Eciiixodermata generally', would lead me to attach but little im- 

 portance to the form of the reticulation as a differential character, since this often varies 

 greatly in the several portions of a single plate ; and Professor Wytille THOiisoif is 

 forced by his reliance upon it to regai'd the superficial cribriform portions of the radial 

 plates and even of the arm-joints as belonging to the perisomatic system, — a supposition 

 which seems to me inconsistent with the fact that the ingrowth of the calcareous reti- 

 culation in the First Radials, by which the axial canal is at first formed as a groove and 

 is afterwards covered in (§ 77), takes place on the cribriform, not on the fasciculated 

 type. In fact there is no distinction in texture between the endogenous additions by 

 which the jirst radials and the hasals are respectively thickened ; so that we cannot 

 place them in separate categories on this score. But further, we have seen that in the 

 stage now described, the Basals as well as the Radials are perforated to give passage to 

 the radiating extensions of the sarcodic axis of the stem, which only reach the radials 

 throilrjli the basals ; so that this ground of distinction also fails to separate them. — I am 

 myself disposed, however, to regard the perforation or non-perforation by the radiating 

 extensions of the Crinoidal axis as quite sufRcient in itself to differentiate the entire 

 skeleton into two series of plates, which, with Professor Wyville THOiisOjN'', I should 

 term the radial and the ])erisomatic ; but I should rank the Basal plates with the 

 former, instead of with the latter. 



85. I shall now describe the changes which each of the component pieces of the skeleton 

 imdergoes in its progress towards the type wliich it presents in the fully-developed An- 

 tedon. 



86. Centro-dorsal piece.. — Through the whole period of growth, the increase of this 

 segment takes place at a greater rate than that of any other part of the skeleton ; so 

 that it soon comes to pass beyond tlie circlet of Basals, and to abut on the proximal 

 edge of the First Radials ; and instead of stopping here, it continues to increase in dia- 

 meter, until it conceals the whole inferior surface of the first radials, and sometimes 



, even encroaches somewhat on the Second (Plate XXXII. fig. 1). AMien we examine 

 into the conditions of this increase, we find them to bear a remarkable resemblance to 

 those which prevail in the growtli of the skeleton of Vertebrated animals. For when 

 we bear in mind the form of the basin, with its cavity and inverted rim (^ 22), and 

 witness its auijmentation in size from its orio-inal diameter of 'OOo inch to its final 

 diameter of 'IG inch, Avith a corresponding augmentation of its cavity, it becomes 

 obvious that there must be not merely a progressive deposit of new material on the 

 external surface, but also a continual removal of old material from^ the internal surface, 

 analogous in every respect to that by which the cavity of a cylindrical bone is enlarged 

 by absorption from within, at the same time as the diameter of its shaft is augmented 

 by the deposit of new layers of bone on its exterior. 



' Reports C'f the British Association for 1S47, p. 117 ct seq. 



