WATEB SYSTEM. 697 



canals, one leading to each ambulacral pore. In the Echinanthidae the am- 

 bulacral tubes of the petaloid part of the ambulacra are fringed, and prob- 

 ably perform the function of gills (PL XI f .f. 17. 18). In a young specimen 

 of Echinanthus the abactinal tentacles could not be distinguished from those 

 of the margin of the test; they were provided with disks, and capable of con- 

 siderable expansion and contraction even after the first appearance of the 

 petals (PL XIf.f. 4). 



In the Petalosticha we find extensive differences in the character of the 

 ambulacral tubes. In the Echinonidae the suckers do not differ from those 

 of the regular Echini ; they are, like them, provided with powerful sucking- 

 disks, from the actinostome to the abactinal system (PI. XIV. f. 2). In the 

 Cassidulidae there are three kinds of ambulacral tubes : the tubes of the 

 phyllodes with lamellar extremities as in Rhynchopygus (PL XXXII. f. 8- 

 10); simple ambulacral tubes, with a slight thickening of the extremity, form- 

 ing more or less rudimentary suckers, — the suckers are well developed in 

 Eehinolampas (PL XVI f. is); they extend from the phyllodes to the base 

 of the petals; — and lastly, the tubes of the petals, which are fiat, broad tubes 

 slightly lobed laterally and at the end (PL XXXII. f. 1;, 7); the tubes of all 

 the petals are identical ; the odd anterior ambulacrum does not differ in 

 structure from the others, the abactinal part of this ambulacrum forming a 

 regular petal, like the lateral ambulacra (PI. XXXIII. f. 2 ; PL XV. f. 

 ,.', 4. J; PI XV. f. 1. ■>). Each one of the tubes of the petals extends, as in 

 Echinarachnius, from the inner pore of the petals to the outer one. 



In the Spatangoids proper we have a still greater variety of ambulacral 

 tubes, owing to the difference in structure of the odd anterior ambulacrum. 

 This is never petaloid in the ordinary sense of the word (PL XXIII. f. 

 PL XXXIII f. 7 ; PL XXXIV. f. 1, s ; PL XXI'. f. 9) ; — see also the fig- 

 ures of Spatangoids seen from the abactinal side (PL XIX". ; PL XIX \ ; 

 PL XIX'.; PL XXI a .; PL XXIII". ), — the ambulacral tubes of the an- 

 terior ambulacrum are usually short, thick, stout tubes, with large terminal 

 suckers (PI. XIX. f. 1, 2), or pointed extremities (PI. XVIII. f. 9, 19). The 

 buccal tentacles immediately round the actinostome terminate in club-shaped 

 appendages (PL XVII f. 10 ; PL XXXII. f. ie). They occupy the figures 

 formed by the pores corresponding to the phyllodes of the Cassidulidae 

 (PI. XXII f. 4; PL XXI f. 4 ; PL XXIII f. 4) (see also figures of Spa- 

 tangoids seen from the actinal side in PL XIX".: PL XIX".; PL XIX C . ; 

 PL XXI". ; PI. XXIII".) which are not as complicated as in the Cassiduloids, 



