VIII 



ECHINODERMATA 



289 



hydrocoele on the other a pair of coelomic canals known as the perihaemal 

 spaces (fh), somewhat triangular in section and separated by a vertical 

 partition. The floor of each of these, which lies in contact with the 

 radial ectodermal nerve> is thickened and each of these thickenings is 

 a coelomic nerve strand {c.n). As they approach the mouth the two 

 perihaemal spaces with their nerve strands diverge from one another and 

 are continued into those of the neighbouring arm. In addition to these 

 the lining of the main coelome of the arm is thickened along its aboral 

 hne and this thickening is also partly nervous though not entirely so, 

 part of it being formed of longitudinal muscle fibres which serve to bend 



rp.c. 



Fig, 119. 



Echinoderm larvae. A, Brittle-star (Ophiura albida) ; B and C, Holothurian {Synapta). A and 

 B are seen from the ventral side, C from the right side. The scales show hundredths of a milli- 

 metre, a, Anus : h, hydrocoele ; l.p.c, left posterior coelome ; m, mouth ; n.s, nervous system ; 

 oes, oesophagus ; p, e.xternal opening of hydrocoele ; r,p.c, right posterior coelome ; sk, skeleton ; 

 St, stomach. The band of cilia is shown in dark tone. 



up the end of the arm as in Fig. 117. In the Crinoids this (" apical ") 

 portion of the coelomic nervous system is greatly developed and controls 

 the movements of the arms. 



The Echinoderms show a very slight development of special sense- 

 organs. In the starfish there are present only eyes, of comparatively 

 simple structure, one at the tip of each arm, upon the swollen base of the 

 single tube-foot in which the hydrocoele ends (Fig. 117, t.t.f). The eye 

 shows in the living starfish as a bright orange-red spot which is exposed 

 to view owing to the tip of the arm being tilted upwards through the 

 action of the muscle already mentioned. Although the eyes are the only 

 definite organs of sense there are present numerous ordinary sensory 



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