10 



ASTEROIDEA. 



On the Homology of the Skeleton in the Asteriadce. 



Although the homology of the skeleton of the Asteriada has long engaged the atten- 

 tion of naturalists, still upon this subject much diversity of opinion prevails, scarcely two 

 of the classical authors holding the same views as to the relation existing between the 

 test of a sea-urchin and the skeleton of a star-fish. Under these circumstances, I purpose 

 giving copious extracts from the works of Delle Chiaje, De Blainville, Miiller and Troschel, 

 Agassiz, Miiller, and Huxley, with the view of placing this interesting subject fairly before 

 the reader. 



Osseous System. 



The inferior part of the rays in the Asteria (says Delle Chiaje),^ or the whole of the 

 rays in the Ophiura, is composed of a series of fragmentary and semicircular bones, 

 almost similar to the vertebrae, the disposition of which deserves a special study 

 (Cuvier, ' Reg. Anim.,' tom. iv, p. 9). The bones placed around the mouth are five in 

 number ; each of these is composed of four articulated parts ; that is, two at the upper end, 

 connected by useful teeth and corresponding hgaments, rounded at. the bottom, and 

 spinous ; each of them, besides, is composed of as many (four) cylindrical, lateral bones, 

 joined with the branches of the other four great vertebrae. 



Thus for each ray there is a series quite decreasing, and each of them is made of two 

 denticulated pieces, provided with ligaments, which have a hole underneath for the passage 

 of the vertebral artery, and besides, of two faces connected with another spinous egg- 

 shaped piece, which shuts the aperture in each ray ; to these are fastened the feet, and 

 in its inside are sometimes found two small annelides, one of which appears to me to be 

 the same as the one described, although roughly, by the illustrious Baster (Opusc. subsec, 

 tom. iv, fig. 9). 



Some other imbricated spines, more or less short, are turned towards the sides of the 

 ray, which at the lateral undermost part terminates in a long articulated spine, near which 

 is found the hole for the passage of sea water, and by another smaller one lodged within 

 the apex. Between this and the vertebra a long piece, according to the amplitude of the 

 ray, articulates itself transversely. As in Astroj^ecten arantiacus, the vertebrae are suffi- 

 ciently large, so the ampullae of the radial arteries fill up the entire space ; whilst in 

 Echinaster ecldnophora the vertebrae being smaller, the ampullae fill up the space alter- 

 nately. The uppermost portion of the ray is found likewise to be formed of a chain of 

 osseous pieces, which are sometimes long and sometimes short. The same conformation 

 is observable in Uraster riihens. 



Besides the file of vertebrae in the rays of Asterias exigua, we notice between each of 

 them many small cuneiform, imbricated bones, formed in the shape of as many triangles 



' Memorie sulla storia e notomia Degli Animali senza Vertebre, vol. ii, p. 289. I am indebted to my 

 friend, M. Ronna, for this translation from Delle Chiaje. 



