28 • ECHINOIDEA. 



connected with specific characters ; some are short or elongated, flattened, cylindrical, fusi- 

 form, or subulate ; others are compressed, spatuliform, or triangular ; others, on the contrary, 

 are expanded, pyriform, or claviform. The surface of the spines is smooth or striated with 

 fine or coarse longitudinal lines; some have verticillate processes at regular intervals, others 

 have asperities, prickles, or granules, disposed with more or less regularity over the surface. 

 The different parts of the spine have received the following names : 



The articular cavity, or acetabulum, is the socket by which the spine articulates with the 

 tubercle ; its margin is smooth or crenulated, according as the summit of the boss is smooth 

 or crenulated (PI. VI and VII) ; in all the genera with perforated tubercles there is 

 a corresponding pit in the socket of the spine, for the attachment of the round ligament 

 which passes from the tubercle to the spine. 



The head is that part of the spine containing the articulating cavity, and is united to 

 the stem near the neck (PI. VII, fig. 4). 



The milled ring surrounds the head ; it is a prominent ridge, more or less deeply 

 crenulated, around which the muscular fibres that move the spine are firmly attached. 



The neck is the smooth space between the line of junction above the milled ring and 

 the rugose body of the spine (PI. VII and VIII). In long, slender, tapering spines, it is 

 often finely striated with longitudinal lines, and cannot be distinguished from the body into 

 which it passes. 



The stem or body of the spine is the part that exhibits the greatest variety of forms, 

 smooth and muricated varieties are figured in PI. I, II, IV. 



The organs of mastication forming " the lantern of Aristotle " are rarely preserved in 

 fossil species; they consist of five jaws, each carrying a long tooth (PL V and VII). As 

 these parts form a complicated mechanism, their analysis and description will be given at 

 length in the anatomical part of the introduction. 



On the relative value of the external organs in the classification of the Echinoidea. 



The mouth is always basal, central, subcentral, or excentral, but the excentricity is 

 invariably towards the anterior border. This opening does not, therefore, afford a charac- 

 ter of primary importance, although, in connection with others, it is valuable in the definition 

 of families. The mouth is sometimes armed with jaws, but it is oftener edentulous. 



The position of the anal opening affords a character of primary importance. In one great 

 section the vent opens within the centre of the apical disc, surrounded by the genital and 

 ocular plates. The relation, therefore, of the digestive organs to those of generation and 

 vision, is an important primary character for the zoologist. In another section the vent 

 opening is without the apical disc, and is more or less external to, and at a greater or less 

 distance from, the genital and ocular plates. The physiological importance of the external 

 relation of the organs of digestion, generation, and vision, to each other, imparts great 



