ECHINODERMA. CRINOIDEA 139 



is called the dorsal cup (fig. 55) ; the part above them is 

 the teg men. The dorsal cup consists at its base of a cycle 

 of five plates, known as basals (figs. 56, b ; 57, c) ; but, 

 owing to fusion, the number of basals is sometimes reduced 

 to four, three, or rarely two. In some forms there is below 

 the basals and alternating with them another row of 

 plates (five or three), termed infra-basals (fig. 57, b), and 

 the base is then said to be dicyclic ; when basals only are 

 present, it is monocyclic. Above the basals, and alter- 

 nating with them, is a cycle of five radial plates (fig. 

 56, r ; fig. 57, d). In some genera there are, between the 

 two posterior radials, other plates, the anal inter-radials 

 (fig. 56, a ; fig. 57, e). 



The arms are characteristic of the Crinoidea; they 

 come off directly from the radials, and are formed either of 

 a single or of a double row of plates, the brachials ; when 

 there is a single row the arm is termed uniserial ; when 

 there are two rows it is biserial. In biserial arms the 

 plates alternate with one another. The dorsal or outer 

 surface of the brachial plates is rounded; on the ventral 

 or inner surface there is a groove in which the soft parts, 

 above described, are placed ; and there is usually also a 

 perforation below the groove, in which the dorsal nerve- 

 cord is situated. The groove in the arms is covered over 

 by a series of plates — the covering plates, which can be 

 opened and closed, and serve for the protection of the soft 

 parts. Where an arm branches, the brachial which sup- 

 ports two branches (figs. 55, 57) has sloping sides, and 

 is known as an axillare. Small unbranched appendages 

 called pinnules occur on the arms of many crinoids (fig. 55); 

 they are similar in structure to the arms, and are given off 

 alternately on opposite sides. In living crinoids the 

 genital products mature in the pinnules. In some forms, 



