PEEMAL SKELETON OF ECHINODER.MA. 203 



continuous with the dermal skeleton, which covers the back of the 

 arms ; at this point there are frequently found one or more longi- 

 tudinal rows of plates or scutes. These structures may be replaced 

 by knobs, and are sometimes continued on to the integument of 

 the antambulacral surface of the body ; or the integument is dis- 

 tinguished by retiform deposits of calcareous matter and smaller 

 tubercles, separated by the non-calcified parts of the perisome. 

 Brisinga resembles the Astero'ida in the structure of its arms ; that 

 is, they have an ambulacral groove. 



Larger flat plates, marginal plates, form the edge of the arms ; 

 these are often distinguished by spicules and other processes. 



The structure of the integument of the Ophiurida resembles 

 that of the Astero'ida. There is seldom any great development of 

 calcareous plates on the antambulacral surface ; they are, as a rule, 

 found near the base of the arm only in these forms. The ambulacral 

 or ventral integument is also provided with plates around the mouth 

 (Fig. 102). But several parts of the firm skeleton of the arms are 

 different to those of the same parts in the Astero'ida. The pieces 

 homologous with the ambulacral plates of the latter form a closely- 

 set series (vertebral pieces, Fig. 100, B w), which almost com- 

 pletely fill up the arm, and only leave a narrow canal on the dorsal, 

 and a groove for the nerves and other organs on the ventral surface. 

 The ccelom is therefore continued into the arms as a narrow canal 

 only. Instead of the soft covering of the ambulacral groove, which 

 the Astero'ida possess, there is a series of firm calcareous scutes 

 (Fig. 100, B b) in the Ophiurida; other lateral processes of various 

 kinds are added on to them. 



In the Euryalida also the leathery investment of the body 

 covers in a skeletal structure, formed of vertebral calcareous plates 

 attached to one another. This, as in the Ophiurida and Astero'ida, 

 belongs to the oral surface of the body ; the plates are continued 

 from the edges along the radii, and to the finest ramifications of 

 them. In them, too, this skeleton forms the floor of the ambulacral 

 groove. On the aboral surface the body-disc is enclosed in a dermis, 

 merely impregnated with calcareous granules, which passes on to the 

 arms, and covers them as far as the edge of the ventral groove. 



There is a larger number of knob-like and spicular processes in 

 the integument, which may vary very greatly in character. A 

 special form is very common among the Astero'ida ; namely, bundles 

 of movable spicules attached to a common stalk (paxillas). The 

 pedicellariae are described in § 166. 



§ 165. 



This dermal skeleton is modified in the Crinoida. The dorsal 

 integument is drawn out into a stalk, to the end of which the 

 animal is attached. The skeleton of the stalk is formed of cal- 

 careous plates, which lie regularly one over the other, and are con- 

 nected with flattened basal pieces, to which other calcareous plates, 



