2 



ASTEROIDEA. 



many genera, but we recommend the study of Tiedemann,^ Meckel,^ Sharpey/ and 

 Miiller's* works on the anatomy of some common species, as examples of what may be 

 achieved in other groups, if correct observation and Uke diligence be brought to the task. 

 As the skeleton of the Asteiioidea is that part of their bodies which is most frequently 



preserved in a fossil state, it is necessary 

 that we should be well acquainted with the 

 structure and relations of its different com- 

 ponent parts. If, for example, we remove 

 from the common star-fish, Uraster ruhens, 

 Lin., the integument and spines from the 

 upper surface of the disc and rays, and 

 afterwards the viscera enclosed therein, the 



Section of a ray of Uraster rubens showing the structure of the ambulacral portion of the 

 arrangement of the calcareous ossieula. skeleton will be well exposed. It is seen to 



consist of a central ring surrounding the mouth-opening, composed of ten larger and five 

 smaller pieces firmly united together by ligaments ; the ten larger pieces are disposed in 

 pairs opposite the base of each ray, and the five smaller pieces occupy the interbrachial 

 angles ; the ten elements of the oral ring are perforated, for the passage of soft tubular organs. 



Each ray is composed of a considerable number of small bones or ossicles, which 

 form rings, as seen in fig. 1, representing the section of a ray of TJraster ruhens, Lin. 

 These bony circles succeed each other from the base to the apex, each segment being a 

 repetition in form and position of all the others ; the size of the rings, however, diminishing 

 gradually from the base to the apex. The ossicles at the under part of the ray [a) are 

 symmetrical, and articulated together in such a manner as to permit of considerable motion ; 

 their upper surface forms the floor of the cavity, in which prolongations of the digestive and 

 other vital organs are contained (c) ; the under surface of the ossicles forms the ambulacral 

 valley through which the tubular suckers pass. In fig. 1 a the two long femur-like bones 

 at the bottom of the ring project obliquely upwards and inwards, and join each other 

 in the median line ; they are articulated at the base with other ossieula, which I shall 

 presently describe. To the lateral parts of this central framework another series (3 h) 

 of larger ossicles are joined, which rise nearly parallel to each other like ribs encir- 

 cling a thorax; they are connected by transverse osseous bars, and the whole is 

 enveloped in the tegumentary membrane which encloses the upper portion of the rays. . 

 The ossieula are lined internally by a white, tough, fibrous membrane, which extends 

 to the sides and floor of the ray, unites the ossicles together, and contributes to form 



' Tiedemann, • Anatomie der Rohrenholothurie des pomeranzenfarbigen Seesterns und Stemigels, 

 Landshut, 1816, folio. 



2 Meckel, ' System der vegleichenden Anatomie,' Band ii, p. 19. 



^ 'Cyclopaedia of Anatomy and Physiology,' Art. Echinodermata, from which figs. 1 and 2 are copied. 

 * Joh. Mdller iiber den Bau der Echinodermen. 1854. 4to. 



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