110 DESCPJPTIOX OF THE HARD PAETS OF SOME 



nearly closing the actinostome, only leaving (PI. XVI. Figs. 2, 3) a small 

 pentagonal opening. 



The interambulacral spines, when the suckers are drawn in close, com- 

 pletely cover the ambulacral furrow (see Fig. 2, PI. XVI., where they 

 are closed over a portion of the ambulacra at the base of one of the 

 arms). 



Pentaceros reticulatus is found on both sides of the Atlantic, at Cape 

 Verde Islands, and in the West Indies, extending north to South Carolina. 

 Several other West India species of Echinoderms are also foimd at Cape 

 Verde Islands and on the main coast opposite. 



In the pentagonal Starfishes the plates forming the so-called jaws are 

 huge interambulacral plates extending far towards the centre of the 

 mouth, where they nearly meet, to form, with the papillae, the so-called 

 jaws and teeth of Starfishes. So far we have not been able in any way 

 to homologize the teeth of Echini with any of the solid parts of Star- 

 fishes or Ophiurans; the auricles of the 'regular Echini and the peculiar 

 spur of the interior of the test near the mouth of some Spatangi being 

 the only processes which appear to have analogous position. For a com- 

 parison of the Starfish mouth parts with those of Ophiurans compare the 

 figures here given with those of Lyman in the Bull. Mus. Comp. Zo()l., 

 Vol. III., from which it is evident that in Starfishes and Ophiurans the 

 mouth parts are strictly homologous, and are formed by the terminal 

 oral interambulacral plates. Comparing profile figures of the oral ex- 

 tremity of one side of an arm in Culcita {^Fig. 7), Acanthaster {Fig. 6), 

 and Solaster {Fig. 8), we cannot fail to be struck with the great size 

 of the terminal oral interambulacral plate aip, carrying the mouth- 

 papilla) mp. 



In the views of the arms, seen from the interior (the abactinal sys- 

 tem being removed), the great development of the oral terminal plate 

 {dc) is well shown. In Fig. 6', Acanthaster, Fig. 8', Solaster, and Fig. 9, 

 Anthcnea, the lettering corresponds to the profile figures. Tlie only addi- 

 tional notation introduced is i p for the interbrachial partition, and i p b 

 for the spur forming the basal plate of the interbrachial partition. The 

 mobility of the arms of Starfishes depends entirely upon the comparative 

 width of tlic ambulacral and interambulacral plates compared in their 

 length, upon the solidity and extent of the interbrachial partition, and 

 the extent to whicli the abactinal system corresponds in its articulation 



