470 COMPARATIVE ANATOMY CHAP. 



directly upon a nerve layer provided with ganglion cells, which, at 

 the edge of the eye, passes into the usual layer of nerve fibres, found 

 in all Echinoids below the surface of the body epithelium. 



Similar spots are found in other Diadematidce, and species of the related genus 

 Astropyga. 



In the Synaptidce (S. vittata), at the base of each tentacle, two pigment spots 

 occur. It has now been proved that these " eyes " are sensory organs, but a detailed 

 account of them has not yet been given. 



XIII. The Body Musculature. 



The special development of the body musculature of the Echino- 

 derms is directly connected with the peculiar development of the 

 skeleton. 



Regarding the musculature and the skeleton alone, the Echino- 

 derms may be divided into three groups. 



Holothurioidea. The skeleton here consists merely of isolated, 

 and usually microscopically small, calcareous bodies. The presence of 

 these in the leathery integument does not prevent changes of form in 

 the tubular body. The body musculature is developed as a dermo- 

 museular tube. By means of co-ordinated contractions of the 

 circular and longitudinal musculature of which this tube is com- 

 posed, the animal is able to make slow, worm-like movements. 



Asteroidea, Ophiuroidea, and Crinoidea. The body in these 

 classes is drawn out in the direction of the radii in the form of 

 arms, which are supported by a jointed skeleton. The dermo- 

 museular tube breaks up into separate muscles, which connect the 

 joints of the skeleton together. In the Asteroidea alone, a dermo- 

 muscular tube persists side by side with these isolated muscles. 



Eehinoidea. The skeleton of the Echinoidea (with but few 

 exceptions) is a rigid test or capsule. A body musculature would 

 here be useless, and is therefore wanting. 



An exception to this rule is found among the Euechinoidea in the 

 Streptosomata, in which the plates of the more or less flexible test 

 imbricate. It has now been proved that in the fichinothuridce, five 

 pairs of muscle lamellae run in meridians from the oral to the apical 

 side on the inner surface of the test. The contraction of these 

 muscle lamellae causes a depression of the test. 



The musculature of the Echinodermata consists, as a rule, of smooth muscle fibres. 

 A longitudinally striated fibril of contractile substance lies on one side of the undiffer- 

 entiated protoplasm of the formative cell which contains the nucleus. Transversely 

 striated muscle fibres are of less frequent occurrence, but are found in the adductor 

 muscles of the seizing pedicellarise in the Echinoidea (p. 398), and in the muscles of 

 the rotating anal spines of Centrostephamis longispinus. 



The musculature of the various organs or systems of organs will be described in 

 the sections dealing with those organs. 



