NOTES ON ECHINODERM MORPHOLOGY. 143 
ambulacral vessels communicates with the exterior. I believe 
this to be the generally accepted meaning of these three 
equivalent terms. 
Now Koehler’s injections passed out from the madreporic 
canal to the exterior; while he describes its inner end as in 
perfect structural continuity with the organ of excretion.1 He 
speaks of “ le canal du sable, qui présente chez les Spatangues 
une structure glandulaire sur une partie de son trajet ;”? and 
the tubular communication of the organ of excretion with the 
oral ring in which the radial (blood-) vessels originate is contin- 
ually referred to by him® as the “canal du sable.” Thus, then, 
the madreporic canal and its continuation through the organ of 
excretion towards the oral vascular ring form the only commu- 
nication between the latter and the exterior, according to 
Koehler’s views. He himself calls one part of this line of 
communication by the French equivalent for “ water-tube ; ”’ 
and I cannot see, therefore, why he is surprised at my supposing 
him to take the same view of that other part of it which lies 
immediately beneath the madreporite, and is described by 
himself as opening to the exterior through its pores. In what 
other way can the following sentence be explained? “ J’étu- 
dierai plus loin les relations du canal du sable avec cet organe 
(of excretion) et sa termination vers la plaque madre- 
porique.”4 
It is just possible that as Koehler does not believe in the 
general independence of the water-vascular and blood-vascular 
systems, the expression ‘‘ water-tube or stone-canal” has not 
the definite meaning for him which is attributed to it by those 
who regard the stone-canal as a part of the water-vascular 
1 § Recherches,’ pp. 98, 99. 
2 Thid., p. 110. 
 Ibid., p. 98. ‘*C, canal du sable,” on pl. i, figs. 1—3 ; pl. ii, figs. 6, 8, 
10,11; pl. ii, figs. 14, 15. 
* Op. cit., p.91. The italics are mine. If Koehler does not consider his 
*madreporic canal” as the water-tube or stone-canal, he would have done 
well not to call it by this name, which has been used by many writers instead 
of stone-canal (Huxley’s ‘ Invertebrata,’ pp. 558, 576). 
