ECHINODERMA. 115 



whole series of each arm into an articulated organ. In some others 

 the faces of the arm-joints are saddle-shaped, and admit of the 

 arms being coiled and twisted, as in Astroschema. These conditions 

 may be known as Streptospondyline. 



The Ophiuroids that possess the simpler streptospondyline type 

 of ossicle never have branched arms, and to them the term Strept- 

 ophiurae may be applied ; those that have saddle-shaped ossicles tend 

 to have branching arms, and may be called Cladophiurae. 



In the great majority of Ophiuroids the extent to which the 

 arm-ossicles can be moved on one another is much reduced by the 

 development of processes and corresponding cavities, which limit the 

 motions of the arm-joints in very much the same way as do the 

 zygosphenes and zygantra of a snake's vertebral column. These, 

 then, are known as the Zygophiurae. 



The principal types of this Order are exhibited in Cases 17-22 ; 

 the most exquisite of them are the forms whose arms are divided and 

 subdivided till they end at last in the finest threads, as in Astro- 

 phyton (481-486), the so-called Basket-fish or Gorgon's heads. 



Ophiacantha vivipara (438) carries its young about with it, 

 and they grow into the adult condition without passing through 

 a free larval stage. 



The ECHINOIDEA (489-711), or " Sea-Urchins," are Echinoderms 

 of a globular, heartshaped, or flattened form in which the rays are 

 not free arms ; the primitive possession of five rays may be seen 

 even in those which, like the Heart Urchins, appear to be bilaterally 

 symmetrical. The calcareous covering generally consists of a series 

 of closely applied plates which form a continuous test ; at the upper, 

 or apical, pole there are five radial and five interradial plates, and 

 five pairs, or more, of calcareous plates are found on the membrane 

 which borders the mouth. 



The Echinoidea are either (1) Regular, when the vent is at the 

 opposite pole of the body to the mouth, or (2) Irregular, when the 

 vent is more or less posterior in position. 



The regular Echinoidea have or have not external gills in the 

 form of five pairs of folded outpushings of membrane set in slits 

 round the margin of the mouth (488, A) ; such as have them are 

 known as the Ectobranchiata. Those in which there are no external 

 gills are the Endobranchiata, and they alwarys have well developed 

 sacs connected with the mouth which appear to be internal gills 

 (488, B), and are called, after their discoverer, the Organs of Stewart. 



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