20G 



COMPAEATIVE ANATOMY. 



Fig. 104. Pedicellaria! 

 of Ecliiuus saxa- 

 t i 1 i s. A A Pedicel- 

 laria with its pincer- 

 arms open ; B With 

 them closed (after 

 Erdl). 



ambulacral canal, and so point to an affinity with this skeleton. 

 The skeleton of the masticatory apparatus in the Echino'ida and 

 Clypeastrida is to be regarded as an independent development ; it 

 surrounds the commencement of the enteron, and consists of a 

 number of calcareous blocks connected together like a scaffold. 



Spine-like processes are connected with the integument in the 

 Echino'ida, as well as in the Asteroi'da, but they 

 are more independent owing to their power of 

 movement. They are articulated to protuber- 

 ances of the calcareous plates, and are provided 

 with a special system of muscles. The spines 

 vary greatly in form and size ; sometimes they 

 are as fine as hairs (Spatangidas) ; sometimes 

 club-like structures (Acrocladia) ; sometimes they 

 are long rods (Cidaris). 



The Pedicellariaa are also dermal organs 

 of a peculiar character, which are found in the 

 Asteroi'da, as well as in the Echino'ida. They 

 consist of a stalk-like muscular process of the 

 integument, which is supported at its end by a 

 fine calcareous skeleton; it terminates in two or three pincer-like 

 valves, which are movable on one another. These too are provided 

 with a calcareous skeleton. In the Echino'ida the three- valved, 

 and in the Asterida the two-valved forms predominate. Brisinga 

 resembles the Asterida in this point. They arc scattered over the 

 whole body ; but in the Asterida they are principally found at the 

 base of the spicules, and in the Echino'ida in the perisome around 

 the mouth. 



These bodies may be regarded as spines modified in such a way 

 that the incompletely calcified stalk of the pedicellaria corresponds 

 to the stalk of the paxilla of the Asterida; the tuft of spinules 



on the latter being represented by the arms 

 of the pedicellaria, which are moved by 

 muscles, just as are the spines of the Echi- 

 no'ida. The four- valved pedicellaria; of Cal- 

 veria fenestrata are intermediate between 

 the more common pedicellaria) and the paxilke, 

 for each of the valves, which is provided with 

 a long stalk, is continued into a broad lamella, 

 bent over at its edges. 



Fig. 105 



anchor. 



A Calcareous 

 B Calcareous 

 plate, which serves to 

 attach the former ; from 

 the integument of S y- 

 napta lappa (after J. 

 Miiller). 



§ 167. 



In the Ilolothuroida the integument has 

 no longer any dermo-skeletal significance. 

 The calcareous plates of the other Echinoderma are represented by 

 disconnected -deposits of lime in the firm dermal layer. 



The calcareous deposits of the skin are definite, and often very 

 regular in form ; and these forms are characteristic both in Synaptse 



