194 LECTURE X. 



the hardening eai-th is present, its ultimate arrangement is a fine 

 network, tlie microscopic meshes or areolae being more or less 

 circular. 



In the genera Oreaster and Culcita the whole surface is beset with 

 tubercles and granules. In Asteropecten and Stellaster there ai'e 

 moveable flat spines and marginal plates. In Solaster and ChcBtaster 

 there are innumerable spines, the summits of which are beset with 

 setos. The sides of the arms in Ophiocoma and Ophiomastix are 

 defended by smooth spines ; in Ophiothrix by bristled spines. In 

 Ophionyx there are moveable double hooks beneath the bristled spines. 



The ultimate muscular fibres in the Asteroids are smooth. The 

 joints of the arms and pinnules of the Crinoids and Comatules are 

 moved by a pair of small muscles on their ventral aspect, antagonised 

 by elastic bands. The spaces between the joints of the skeleton of 

 the Asteroids are occupied by muscular fibres, which ai"e antagonised 

 by the general elasticity of the integument. In the Crinoids the 

 margins of the ventral grooves extending from the mouth along the 

 soft perisome are beset with very delicate cylindrical muscular feet, 

 and the surface of each foot is beset with small clavate tentacules. 

 In the OphiuridcB at the sides of the arms there are poriferous plates, 

 out of which protrude cylindrical obtuse feet, covered by a quantity of 

 wart-like protuberances. In this family the rays are extremely 

 attenuated and elongated, and have no ambulacral grooves : nor is 

 the complicated meclianism of the ambulacral feet in the ordinary 

 star-fishes here needed, for the flexile and spinous rays can twine 

 round and seize other objects so as to perform directly the oifices of 

 prehension and locomotion. The facility with which an Ophiura or 

 Luidia casts off a ray which may be touched, and even all the rays, 

 leaving only its central disc, when it is seized, is very surprising ; it is 

 consequently very difficult to preserve specimens of these genera 

 entire. To do this it is recommended to plunge them suddenly 

 into fresh water, when they instantly die in a state of the most rigid 

 extension. 



According to MUller there extends along the tentacular groove of 

 the perisome on the ventral side of the body of the Crinoids a nervous 

 chord, which forms a slight enlargement at the base of each pinnule, to 

 which a filament is sent. In the OphiuridcB the nervous chords are 

 lodged in a canal protected by the ventral plates of the arms. The 

 soft labial membranes, tentacles, and tubular feet seem adapted to a 

 delicate reception of impressions ; and so far as these may be felt by 

 the individual, and cause voluntary movements, such parts may be 

 regarded as organs of touch. The nervous system of the Asterias 

 ( p. 14., Jig. 4..) consists of a slender white chord surrounding the 



