THE HOLOTHURIOIDEA 223 



left respiratory tree are a large number of long slender pro- 

 cesses, the Cuvierian organs (Fig. I. 1, c.o). These remarkable 

 organs, formed of spirally coiled fibrous tissue covered with a 

 layer of modified coelomic epithelium which secretes a sticky 

 substance, are shot out of the anus through the wall of the cloaca 

 by the living animal when irritated. The organs swiftly elongate, 

 forming adhesive white threads, to which this Holothurian owes its 

 name of Cotton-spinner. 



The blood-vascular system consists of a circular vessel round 

 the oesophagus, giving off' five radial vessels which run between 

 the water-vascular canal and the pseudhaemal canal, covering the 

 radial nerve internally. The anterior region and genital organs 

 are supplied from the circular vessel. The alimentary canal is 

 provided with two longitudinal trunks coming from the circular 

 vessel one, the "dorsal" or mesenterial vessel, runs along the 

 region where the mesentery is attached to the intestine ; the other, 

 the " ventral " or antimesenterial, runs along the alimentary canal 

 on the opposite side (Fig. II. 1 and 5, m.v and a.v). A cross 

 vessel passes from the mesenterial vessel on the first region of 

 the intestine to the antimesenterial vessel on the second region. 

 In connection with the mesenterial vessel (especially along the 

 second region of the intestine) is an extensive and rich plexus of 

 blood-vessels, the rete mirabile, overlying the left respiratory tree 

 (Fig. I. 9). Blood lacunae extend in the walls of the alimentary 

 canal, respiratory trees, gonads, and other parts. 



A closed pseudhaemal system extends over the inner surface 

 of the nerve ring and radial nerves. 



The body -cavity forms a continuous coelomic space lined 

 throughout by epithelium, which is generally ciliated. 



The sexes are separate in this species. The genital organs consist 

 of a bunch of tubes, with free blind ends lying in the coelom, and 

 uniting at their base to open into a duct running forwards in the 

 dorsal mesentery (Fig. II. 1 and 5, g and g.d). The genital duct 

 opens by a median dorsal pore behind the tentacles (5, g.-p). 



Variation in the Holothurioidea. In outward form the 

 more specialised and highly modified genera depart very widely 

 from the elongated and somewhat pentagonal shape which appears 

 to be the more primitive. In organisation, however, this group is 

 fairly constant, and clearly denned from the other classes of Echino- 

 derma. The epidermis is not ciliated in the adult. The body-wall 

 is remarkable for its thickness and leathery consistency, the cal- 

 careous skeleton being rarely in the form of plates or scales, and 

 more often as minute spicules of various shapes (there is no apical 

 system of plates). When these spicules are of different kinds they 

 generally form distinct layers in the cutis as, for instance, in 

 Holothuria impatiens, where the " tables " lie on the surface, whilst 



