178 



INVERTEBRATE ZOOLOGY 



STone canct^-K 

 Esophagus- 



Geniralduct 



Si'omach 



Gonad 



Calcareous ring 

 Retracf or muscle 

 Polian 

 vesicles 



a circle of branched tentacles {Y\g. 87) and an anus at the opposite 

 extremity. Since the mouth end goes forward in locomotion, 

 it is frequently called the anterior extremity in the holothurians. 

 Typically, the body is five sided and on each side bears a double 

 row of tube-feet. In some species, the three sides constituting 

 the ventral surface have the tube-feet more highly developed than 

 they are on the two dorsal ambulacral areas. Some few forms 



have feet irregularly scat- 



tered over the body sur- 

 face, while some burrowing 

 species lack feet. The 

 body wall is highly muscu- 

 lar. The alternate use of 

 longitudinal and circular 

 muscles enables the cucum- 

 ber to creep like a worm. 

 Though there is no con- 

 tinuous skeleton, the body 

 wall is rather firm. This 

 is due in large measure to 

 the presence of microscopic 

 calcareous plates embedded 

 in the tissues. The form of 

 the plates is highly variable 

 in different species and they 

 serve as important features 

 in classification. In some 

 species, a calcareous ring 

 of ten plates surrounds the esophagus and serves as a support 

 for the tentacles. In a few forms, Psolus for example, the 

 body is encased in hard scales or plates. 



The ring canal of the ambulacral system is located around 

 the esophagus just behind the tentacles. From the ring canal the 

 radial canals pass to the posterior extremity of the body. In the 

 first part of their course, they run anteriorly and give off branches 

 to the oral tentacles which are in reality highly modified tube-feet. 

 One or more Polian vesicles are frequently present. The stone 

 canal, instead of opening to the exterior, bears one or more 

 madreporites which open into the coelom. As in other echino- 

 derms, the water-vascular system is rather closely paralleled by a 

 blood-vascular system the vessels of which form extensive 



'esetrtert) 



Respirahrij 

 free 



- Muscle 

 Cloacaf muscles 



Cloaca 



Anus 



Fig. 87. 



-Anatomy of a holothurian, Thyone. 

 {After Coe). 



