16 



BRITISH PALAEOZOIC ASTEROZOA. 



on tlie surface over \vliich tliey are moving. The tube-feet act mostly as respira- 

 tory or sensory organs. 



The homologies of the component skeletal parts have been established by 



10 



S. Sh. 



11 



V. Sh.. 



12 



J. 3 12 



-.<^'' *: r 



8.8h. 



D.Sh. 



J'.SK -S.Sh. 



g.s. 



Text-fig. 10 (on left). — Apical view of portion of Ophiothrix cxspitosa. D. Sh., dorsal shield; 8. Sh., side 



shield. 

 Text-fig. 11 (in middle). — Side shield of same species showing spines. 

 Text-fig. 12 (on right).— Oral view of same species. S. Sh., side shield; V. Sh^., first ventral shield ; V. Sh^,., 



sixth ventral shield ; J., jaw ; </. s., genital slit : 1, lateral buccal shield ; 2, buccal shield ; 3, mouth slit. 



(All after Lyman.) 



Johannes Miiller, Ludwig and other workers. They can, perhaps, be most readily 

 understood from the diagram given of a cross-section through the arm (Text- 

 fig. 13). The ambulacral channel is seen to lie at the base of the vertebrge. 

 There is in consequence no difficulty in recognising that the vertebrtB occupy the 



J^S/i. ep.c Mm.CA. 



Text-fig. 13. — Diagrammatic cross-section of the arm of an Ophiuroid. D. Sh., dorsal shield; 8. 8h., side shield; 

 V. Sh., ventral shield; ilf,., dorsal longitudinal muscles; 31,,., ventral longitudinal muscles; Am. Ch., 

 ambulacral channel ; coe., coelom ; ep. c, epineural canal. 



same position as the paired ambulacralia of the Asteroidea. Although the 

 vertebrae of the adult are solid unpaired ossicles, in the young they arise by the 

 fusion of paired components, and thus complete the homology established on 

 purely anatomical grounds. 



The side shields, alone of all the sheathing plates, articulate with the vertebrjB 

 (Text-fig. 15). They are really the adambulacralia. The mesenchyme from 

 both sides has grown over and united so as to close the ambulacral groove, 



