318 



COMPARATIVE ANATOMY 



CHAP. 



probable that certain pieces or plates of the skeleton are homologous 

 in all the divisions. It may be assumed that these plates composed 

 the primitive Echinoderm skeleton. From this primitive arrange- 

 ment, the skeletons of all known Echinoderms, whether extant or 

 extinct, appear to be derived, on the one hand, through the loss of 

 certain pieces of this primitive skeleton, and, on the other hand, by 

 the acquisition of new or secondary pieces of varied form, number, and 

 arrangement. 



The hypothetical primitive Echinoderm skeleton consists of two 

 principal groups or systems of plates : (1) the oral, and (2) the 

 apical. 



The oral system has five interradially placed oral plates, 

 arranged radially round the oral pole. This oral system develops 



round the left ccelomic vesicle, 

 out of which the oral portion 

 of the coelom rises. 



In the apical system the 

 following plates occur: (1) a 

 ^^ central plate at the apical 



"^mS^^^J^^y^ P ole; ( 2 ) a circle of five 

 Mw/P^plBr \>J3 radially placed plates, the 

 |Kf^f3||P-^ infrabasals; (3) alternating 

 \^K. ^h^^JV^^i^ with these, five interradially 



placed plates, the basals ; and 

 (4) around these, five radially 

 placed plates, the radials. 

 The apical system develops 

 on the right ccelomic vesicle, 

 from which the apical portion 

 of the ccelom is derived. 



The stalked larva of 

 Antedon (Fig. 270) has 

 retained this system of plates 

 less altered than in any 

 known Echinoderm. It has all the typical pieces of the oral and 

 apical (or aboral) system of plates. 



All the skeletal plates of the Echinodermata consist of carbonate 

 of lime. Their microscopical structure is very characteristic, so that 

 small fragments can at all times be recognised and distinguished 

 from similar fragments belonging to the skeletons of other animals. 

 The structure is a sponge-work ; and thin sections of the skeletal plates 

 or of the microscopic calcareous bodies appear to be perforated in a 

 lattice-like manner. The finer structure, especially of the spines of 

 Sea-urchins, is of great systematic importance. 



FIG. 270. Diagram of the apical system of the 

 Antedon larva, combined from various stages. Ex- 

 planation of the lettering on p. 317. The number of 

 infrabasals is here shown as 3, but these are produced 

 by fusion of 5, which number has also been seen. 



