REPORT ON THE ECHINOIDEA, 87 



Young specimens of Asthenosoma pellucidum (PI. XVIII. figs. 1, 2) show how close is 

 the relationship between the genera Phormosoma and Asthenosoma in spite of the 

 apparently great structural differences existing between the adult of such species as 

 Asthenosonui grubii and Phormosoma luculentnm. It is mainly from the comparatively 

 larger number of coronal plates in the former genus that the young of the two genera 

 can Ije satisfactorily distinguished, the other characteristic features, the lapping of the 

 plates appearing only in larger specimens. There is nothing in the growth of 

 Asthenosoma corresponding to the splitting up into separate plates of the primary 

 coronal plates so characteristic of Phormosoma when seen from the interior of the test 

 (PI. XVIII.» figs. 4, 5, 7, 8). 



Each primary plate of Phormosoma, consists of a number of plates most irregular 

 in shape (PI. XVIII. " figs. 4, 7), held together quite loosely by the inner integu- 

 ment of the test, the joints of the plates are often foliated, and the centre is 

 sti'engthened by a thicker deposition of calcareous matter forming a sort of button ex- 

 tending beyond the level of the plate towards the interior of the test. This thickening 

 which corresponds to the lower side of one of the primary tubercles I have described 

 more in detail in the description of Phormosoma tenue. This species difiers from both 

 Asthenosoma hystrix and Asthenosoma fenestratum in having a smaller number of coronal 

 plates, these are consquently higher ; in Asthenosoma jfellucidum this difierence becomes 

 very striking in the height of the plates of the ambulacral areas. The open spaces between 

 the coronal plates are largest on the actinal surface where they appear first in the 

 younger stages and gradually extend to the abactinal surface with increasing size. In a 

 small specimen measuring 36 mm. in diameter, the coronal interstices are limited to a 

 narrow line parallel to the edge of the plates. In specimens measuring 44 mm. the coronal 

 interstices assume already the elongated form represented in Plate XVIII. fig. 4. In 

 the smallest specimen of this species examined, measuring 36 mm. in diameter, the gills 

 are reduced to a small forked appendage protruding between the edge of the coronal 

 plates and the mailed actinal memlirane. In the older specimens of Asthenosoma 

 pellucidum there is a decided thickening of the epidermis of the test, which tends gradually 

 to obliterate the outlines of the coronal plates. We have this character developed to a great 

 degree in Asthenosoma coriaceum. Unfortunately, the largest specimens oi Asthenosoma 

 pellucidum are so much smaller than the smallest Asthenosoma coriaceum or the single 

 specimen of Asthenosoma tessellatum, that I am unable to satisfy myself that the present 

 species (Asthenosoma pellucidum) may not be the young of Asthenosoma coriaceum. In 

 the only species of the group of which the Challenger collected a complete series (Phor- 

 mosoma tenue) there was little difficulty in recognising the young as belonging to the 

 adult, the same was the case for Phormosoma luculentum. The changes in the coronal 

 plates are not as great, and the arrangement of the pores does not seem to vary as much 

 in Phormosoma as in Asthenosoma. 



