39 



The terminal joint of the arm is however distinguished', as stated, from the others by a 

 remarkable development, and by the absence of water-feet, in stead of which it is furnished 

 with the above-noticed special organ of sense. This joint is, as will be shewn hereafter, 

 the oldest of them all; and it is from the basis of this that all the others develop them- 

 selves little by litle during the growth of the arm. There is a common cuticle for all the 

 metamera and a common blood-cavity. Of the interior organs contained in the latter, the 

 digestive system (the radial caeca) is limited to the basal third part of the arm; and the 

 organs of generation are limited only to a certain section of this last part. 



If we now endeavor to picture to ourselves the manner in which these different 

 organic systems are topographically arranged in relation to each other, we shall obtain the 

 best view by examining a section of an arm. If we take this in the basal part, and at the 

 place where the organs of generation are situated, we shall find all the most important 

 organs of the arm' in the section and in their relative positions, (see Tab. Ill, fig. 3). Above 

 and on the sides there appears the highly convex dorsal cuticle with its 2 layers; on the 

 outside, the soft epidermoidal layer with its pedicellariaB ; on the inside, the fibrous layer 

 with the calcareous ribs contained therein, from which rise the spines ranged in circlets 

 along the same with their cuticular sheaths formed by the epidermis. Limited above and 

 on the sides immediately by the cuticle, appears the perivisceral cavity or the blood-cavity 

 with the interior organs inclosed; highest up and intimately connected with the inside of 

 the cuticle, the double radial caeca (d d) with their lateral sinuosities ; further down and on 

 the sides, the well developed organs of generation (e e) which here fill the greater part of the 

 perivisceral cavity, with their numerous ramified caeca; below, and here limiting the peri- 

 visceral cavity, appears the ambulacral skeleton (b) with the dorsal ridge (projecting upwards 

 in the middle into the interior of the arm), on the sides of which ridge the thin-skinned 

 ampollse (e) for the water-feet project from the ambulacral pores. The ambulacral skeleton 

 is connected on each side with the dorsal cuticle, and thus closes completely the cavity of 

 the arm below; the long marginal spines (ff) project externally at this place on each side. 

 Immediately below the ambulacral skeleton, at the bottom of the ventral furrow formed by 

 the same, there appears in the middle a narrow opening (see fig. 1 b) which is the section 

 of the radial ambulacral vessel.; this opening is in the lower part separated by transversal 

 ligaments from another narrow opening (fig. 1 a) which represents the radial blood-sinus; 

 this last is again covered by the band-shaped radial nerve, which nerve again is covered by 

 the thin membrane lining the ventral furrow and going over at the sides immediately into 

 the tendinous cuticle which clothes the ambulacral skeleton. From the bottom of the ambu- 

 lacral furrow there project to a greater or less distance the 2 water-feet (fig. 3 a) and from 

 the adambulacral plates which limit the ventral furrow on the sides, there issue in various 

 directions the so-called furrow-spines. 



By examination of a section of the disc (see "Fab. VI, fig. 36) we may ascertain that 

 the various connecting parts of the organic systems have on the whole a similar arrange- 



