THE ECH1NOIDRA 285 



(1841). The next important advance was made by the memoirs of 

 Joh. Miiller (1850-57) who gave a full account of Cidaris. Among 

 later general contributions to anatomy may be mentioned those of 

 Teuscher (1876), Koehler (1883), Prouho (1887), Fredericq (1876), 

 Perrier (1869 and 1875), Hamann (1889), and the last part 

 of A. Agassiz's Revision. Special types have been described in 

 separate memoirs, such as Cidaris and its internal branchiae, by 

 Stewart (1880); Spatangus, by Hoffmann (1871); Asthenosoma, 

 by P. and F. Sarasin (1888); Phormosoma, by Bell (1889); 

 Dorocidaris, by Prouho (1888) ; Pourtalesia, by Loven (1883). 



Knowledge of the anatomy of the skeleton is due to many 

 students, but especially to the masterly series of memoirs by 

 Loven. The structure of the spines has been described and their 

 taxonomic value shown by Mackintosh (63) and A. Agassiz (1, 2) ; 

 the fascioles were first used in classification by Liitken. The pedi- 

 cellariae Avere first described by 0. F. Miiller, who regarded them 

 as parasites, a view disproved by Delle Chiaje ; L. Agassiz con- 

 sidered them to be young Echini, which was shown to be erroneous 

 by Valentin's detailed figures. An attempt to use pedicellariae as 

 a basis for classification was made by Perrier (1869-70); their 

 function has been the subject of a long controversy. 



The study of the embryology of Echinoidea was begun by Derbes 

 in 1847, and by Miiller in a series of memoirs (1848-55); it was 

 continued by Krohn (1849-57), A. Agassiz (1864), Metschnikoff 

 (1868-69), Bury (1889 and 1895), Theel (1892), MacBride 

 (1896), and others. 



The term Echinoidea, now used as the name of the class, was 

 applied by Aristotle to animals that resembled Echinus, which has 

 always been regarded as the most representative genus. Though 

 in some ways rather complex, it may be conveniently studied as 

 a Typical Sea-Urchin, since specimens can be easily obtained. 



The Skeleton. If we examine a specimen of the common 

 British species Echinus esculentus, we first notice that it is 

 covered all over by short, bluntly pointed spines, which are 

 coloured violet at the tips. If we pull off the longest spines, 

 we find between them a number of smaller " secondary " spines 

 (compare Fig. XXXIII.). After removal of all the spines, the 

 general character of the main shell can be seen. It is com- 

 posed of closely fitting plates, which together form the rigid 

 "test" or shell. The shape is rounded in transverse section; 

 the surface on which the mouth opens is flattened, while the 

 upper half of the test may be either well rounded or sub-conical. 



On the centre of the lower flattened surface there is a large 

 flexible membranous area covered with loose plates which bear 

 short spines. This is the " peristome " ; the mouth opens in the 

 centre of this space ; its exact position can be determined by 



