270 THE VOYAGE OF H.M.S. CHALLENGER. 



general, the arm plates that recall Ojyhiomyxa; nay, one Astrophyton character, the ad- 

 hesion of the digestive cavity to the disk wall, is carried farther than in Astrophyton itself. 

 In conclusion, it is jn'oper to point out a slight resemblance which the branching 

 Astrophytidje have to the order of Starfishes. This is in the pouches of the digestive 

 ca\dty filled with a clotted matter, which suggest the varied ctecal appendages charac- 

 teristic of difi"erent genera among Asteroidea. 



Species of Astrocnida not herein described, 



Astrocnidci isidis, Lym. (PI. XLVI. fig. 2). 



Astrocnida isidis, Lym., Ann. Sci. Nat., 1872, p. L 



Trichaster isidis, Duchassaing, Animaux Eadiaii'es Jes Antilles, 1850. 



West Indies ; 56 to 120 fathoms. 



Astrop)Oi'2)a- 



Astroporpa, Orst. & Ltk., Vid. Meddel., Jan. 1856. 



Disk divided into five radiating wedges by the pairs of radial shields and covered, 

 together with the arms, by a minute pavement of fiattened granules, which below are 

 more scattered and rounded. The arms are simple, and their under surface is almost 

 covered Ijy the side arm plates which bear several rough cylindiical, tentacle scales (or 

 arm spines). These plates are continued upward by a narrow, regular double line of 

 close-set nodules, homologous with upper arm-plates, and bearing minute hooks. They 

 thus form very regular raised belts on the arms, and are continued over the back of the 

 disk as irregular concentric cii'cles, of which the outer ones are hook bearing, like those 

 of the arms, and those nearest the centre have often microscopic thorns, the remains of 

 hooks. Two small genital openings in each interbrachial space at the outer corners. 



Here is an excellent illustration of the homology between tlie roof of the disk, and 

 the top and sides of the arms, or of the fact that the skin of the latter is a prolongation 

 and a bending down of the skin of the roof. Seen from above, an Astrop>orp)a looks as 

 if composed only of five arms, whose bases were thickened and wedged together to form 

 the disk, an effect heightened by the continuation of the hook-bearing ridges from belts 

 on the arms to concentric circles on the disk. Astrogomphus has in place of them 

 rather irregularly disposed spikes, while Astrocnida has pretty well-marked circles, 

 some of which bear hooklets, while others are simple spikes. 



Species of Astroporpa not herein described. 



Astropor2')a anmdata, Orst. & Ltk., Vid. Meddel., Jan. 1856, p. 17 ; Addit. ad Hist, 

 part 2, p. 152. 



Astroporpa dasijcJadta, Diij. & Hup6, Hist. Nat. Zooph. EcLin., p. 298, 1862. 

 Barbadoes; 100 fathoms. West Indies ; 50 fathoms. 



