150 Introduction to Animal Morphology. 



are the trees singly branched (Echinocucumis). The 

 pseud-haemal ring sends off two pulsating vessels along 

 the intestinal canal, dorsal and ventral, connected 

 by reticular vessels; branches pass from one of these 

 to the right water-lung, and in Aspidochirotae a plexus 

 of branches of the dorsal vessel is related to the left 

 respiratory tree. The ventral vessel in Chirodota dis- 

 plays numerous varicosities. The yellow or brown 

 blood is coagulable, and contains colourless corpuscles 

 and amaeboid mucous cells. 



The intestine is lined by yellow cells, secreting a bitter 

 fluid (bile ?). Solid fibres or thread-like tubes open into the 

 cloaca (Cnvierian organs). They may be as long as the re- 

 spiratory tree, and cxcal (30, Bohadschia), or small (Holo- 

 thuria), filiform, grape-like, and very numerous ; or bundles 

 of spindle-shaped saes (Mulleria, Pentacta), filled with fine 

 vesicles. They are absent in Apneumona, &c. The body 

 surface often secretes slime from its dermal follicles. The 

 nerve-ring is often red, thicker than the radial branches, and 

 lies in front of the calcareous plates and water-vascular ring; 

 it sends off tentacular and ambulacral nerves, the latter some- 

 times trifid, through holes in the radial plates. Inter-tenta- 

 cular pigment flecks in Synapta are described as ocelli, but 

 no nerves have been traced satisfactorily into them. Synapta 

 Heselii has small vesicles placed around the mouth where the 

 radial nerves pass through the holes in the oral plates, 

 possibly otocysts. The spicules in the tentacles usually differ 

 from those of the body. 



The ambulacral oral circle may have i-ioo (Cla- 

 dolabes) Polian vesicles. The stone-canal is short, 

 usually single, but may be multiple (Synapta serpen- 

 tina), or branched (S. Beselii). It hangs free from the 

 body cavity, often holding a wide, calcareous, spiculi- 

 gerous sac with a sieve-like wall at its end, represent- 

 ing the madreporiform plate. This canal has often a 



