PHORMOSOMA PLACENTA. 109 



Phormosoma placenta Wyv. Thom. 



Phormosoma placenta Wyv. Thom., Trans. R. S. London, Vol. 164, Part 2, 1S74, p. 719. 



Plate 43, figs. 1-4 ; figs. IS, '"Blake" Station 100, off Nevis, 375 fathoms; 



figs. 3—4, " Blake " Station 29, off the Tortugas, 955 fathoms. 

 Plate 48. fig. /, "Blake" Station 150. 



In a young Phoiinosoma placenta 7 mm. in diameter (PI. 43, fig. £) the 

 genital ring is composed of pentagonal genital plates, the madrepoiite 

 genital and the left anterior are somewhat larger than the others and 

 carry from one to three large miliaries; the madreporic body can barely 

 be detected. 



The ocular plates are hexagonal, they extend between the interambulacral 

 zones to the level of the second abactinal plate; they project much less 

 along the coronal plates than they do in a somewhat older stage (PI. 13, 

 fig. 4), 9 mm. in diameter, in which the proportions between the oculars and 

 genitals, joined on smaller sides, are similar to those of a somewhat older 

 stage of Phormosoma hispidum (PI. 40, fig. 1). The anal system of each 

 of these stages (PI. 4o, figs. .', 3) is pentagonal and covered with rows of 

 plates very irregular in size and shape. 



The actinal system of the specimen 7 mm. in diameter is marked for the 

 great size and elongated shape of the ten original buccal plates at a time 

 when there are only three rows of actinal plates. The great depth of the 

 primordial plate is also characteristic as compared with that of stages of 

 other species only slightly older (PI. 43, fig. 5). In a somewhat older stage 

 the first row of buccal plates has lost its great preponderance (PI. 43, fig. 

 • -'), and the second row is now by far the most prominent. On the same 

 figure a number of spheridia are figured both on the ambulacral and 

 actinal plates. 



In the young specimen of Ph. placenta of 7 mm. there are eight inter- 

 ambulacral plates (PI. 48, fig. /). All the ambulacral plates reach the inter- 

 ambulacral edge. On the actinal side all the primary plates, except one, 

 reach the median line, while on the abactinal side the second and third com- 

 ponent plates occupy but a small angle in the first primary plate, until we 

 come to the primary abactinal plates of the ambulacrum where they again 

 all reach the median interambulacral line. 



