232 THE HOLOTHURIOIDEA 



of Cucumaria and Psolus ; in other cases several tentacles may be small, 

 and these may even form an inner ring surrounded by an outer ring 

 of large tentacles, as in Phyllophorus and Pseudocucumis. The stone- 

 canal may be single or multiple. Five powerful retractor muscles, reach- 

 ing from the body -wall to the radial pieces of the calcareous ring 

 (Fig. IV. 1 and 5, r.m), serve to invaginate the anterior region of the body 

 so as to withdraw the mouth and tentacles out of sight (Fig. IV. 5). The 

 body-wall in the anterior region is thinner and often devoid of podia 

 (Fig. IV. 1 1). Every stage is exhibited between the arrangement of the 

 podia in a double row along the radii, as in Cucumaria pentactcs 

 (Fig. IV. 4), and their distribution over the interradial areas, as in Thyone 

 fusus (Fig. IV. 11). In Theelia they are nearly, and in Psolus entirely, 

 reduced on the "dorsal" surface. This is correlated with the develop- 

 ment of a very distinct creeping sole, to which the podia are restricted 

 (Fig. IV. 10 and 8). In these genera the calcareous deposits in the dorsal 

 region form large plates or scales ; five triangular so-called " oral " plates 

 close over the introverted anterior region, and small plates may surround 

 the anus. In Colochirus and Actinocucumis somewhat similar valves are 

 developed in front. Although the spicules may be in the form of large 

 perforated plates, sometimes produced into spines, as in Sphaerothuria 

 bitentaculata, Ludwig (Fig. IV. 9), they are more usually rods or knobbed 

 buttons and plates (Fig. IV. 2). 



The calcareous ring in the Cucumariidae is very well developed, the 

 pieces being large, and frequently made up of several plates fitting 

 together. The radial segments, to which the retractors are attached, are 

 generally produced backwards into two long processes (Fig. IV. 6). A 

 more or less pronounced bilateral symmetry is often brought about by 

 the unequal development or fusion of the pieces. Cuvierian organs 

 have been described in Cucumaria frondosa and C. nigricans. The genital 

 tufts are paired, and load into a duct which usually opens to the exterior 

 on a papilla within the circlet of tentacles (Fig. IV. 1 and 3, g.p). 



In Sphaerothuria (Fig. IV. 9) the dorsal surface is reduced, the 

 mouth and anus being approximated and the ventral radii much curved ; 

 this process is carried to an extreme in the extraordinary genus Rho- 

 palodina (Fig. IV. 7), where the body has assumed a flask-shape, the 

 mouth and anus are close together at the small end, while the radii are 

 bent round. This appearance misled the early observers, who described 

 Rhopalodina as having ten radii. 



Whilst the Cucumariidae resemble the Holothuriidae in the possession 

 of well-developed respiratory trees, they differ markedly from all the 

 preceding families in the shape of the tentacles and the presence of 

 retractor muscles. 



FAMILY 5. MOLPADIIDAE. Neither tube-feet nor papillae. The 

 posterior region generally tapering. Mouth and anus terminal. Generally 

 fifteen simple or digitate tentacles. Tentacle ampullae well developed. 

 Calcareous ring of five radial and five interradial pieces. Single stone- 

 canal with an internal madreporite. Longitudinal muscle bands more 

 or less paired. Respiratory trees present; Cuvierian organs rare. 

 Genital tubes in right and left tufta Genera A. With well-developed 



