49G THE MICROSCOPE. 



breadth of the rays. A striking species is the Pahnipes 

 membranaceus, the " Bird's-foot Sea-Star," which is almost 

 as thin as parchment, and might, as Professor Forbes says, 

 be readily mistaken for the torn-off skin of some bulkier 

 species. Its surface is covered with a number of raised 

 tubercles, and very closely-set fasciculi of short and sharp 

 spines. Asterias aurantiaca and Luidia fragilissima 

 present a surface-structure very different from any of the 

 species previously noticed, their tuberculated epidermis 

 being so closely set with upright spines as to be almost 

 wholly invisible. These spines are arranged in a radiated 

 or rosette-shaped manner, and have a roughened surface. 

 A portion of the ray of Luidia forms a microscopic object 

 of exquisite beauty, A single spine is given in Plate IV. 

 Xo. 89. 



The cirrhigrade star-fishes are furnished with certain 

 curious appendages, the use of which is at present very 

 imperfectly understood. These are the " pedicellariw" and 

 " madreporiform tubercle." The latter is a rounded, 

 cushion-like eminence of considerable size, situated on the 

 disc, mostly very much out of the centre. It is irregularly 

 fissured in a radiate manner, and it is not at all unlike the 

 animal from which it derives its name. Various conjec- 

 tures have been made as to the use of this tubercle. 

 Forbes looks upon it as being merely the analogue of the 

 stalk which exists in the young condition of the crinoid 

 star-fishes. The pedicellarice (Plate IV. Nos. 93 and 94) 

 are pincer-like organs irregularly scattered over the surface 

 of the animal, and which have distinct characters in the 

 different species. They were supposed to be parasitic 

 creatures, but are now generally admitted to be true epider- 

 mic appendages. They are in a constantly active motion 

 during the life of the star-fish, and grasp firmly anything 

 which is brought between their blades. Their nearest 

 analogues are the birds'-head processes which occur in 

 certain zoophytes. The Pedicellarice of Echinus are par- 

 tially covered with ciliated epithelium : they are also 

 placed upon a stalk, the lower portion of which encloses 

 a calcareous nucleus, whilst the other portions are soft, and 

 spirally retractile. 



The Feather-star (Comatula rosacea) is perhaps the most 



