230 THE HOLOTHURIOIDEA 



and the absence of well-developed respiratory trees (pp. 222, 225), yet 

 they are undoubtedly closely related to them, and more especially to 

 the Synallactinae. 



FAMILY 3. PELAGOTHURIIDAE. Body cylindrical, and produced at the 

 base of the crown of tentacles into an umbrella-like disc, drawn out 

 into long slender rays. Mouth and anus terminal. No podia ; branches 

 of the tentacle canals extending into the rays of the disc. No retractors 

 and no respiratory trees ; a single stone-canal opening to the exterior. 

 Right and left tufts of genital tubes. No calcareous skeleton. Genus 

 Pelagothuria, Ludwig (Fig. III. 7 and 8), represented only by a remarkable 

 free-swimming Holothurian, Pelagothuria natatrix, recently discovered in 

 the Pacific. The thirteen to sixteen tentacles are forked and beset with 

 papillae, and the disc is produced into a corresponding number of rays, 

 each containing a branch of the tentacular canal. These rays may repre- 

 sent the modified tentacular ampullae of the Holothuriidae ; or the disc 

 may be derived from the anterior brim, which occurs in the two preceding 

 families. The five radial vessels are normally developed, in spite of the 

 absence of podia. The longitudinal muscles are in simple bands. Neither 

 calcareous ring nor calcareous spicules are developed. 



This highly modified form undoubtedly belongs to the Actinopoda, 

 since there are radial canals, and the circular muscles are interrupted 

 at the radii. The absence of respiratory trees, and of free tentacular 

 ampullae, and the simple longitudinal muscles remove it from the Mol- 

 padidae ; whilst the first of these characters, combined with the absence 

 of podia and retractors, separate it from the Cucumariidae. With the 

 Elpidiidae, on the other hand, it has many characters in common, such 

 as the single stone-canal opening to the exterior, the simple longitudinal 

 muscles, the absence of respiratory trees, and the reduction of free ten- 

 tacular ampullae and the calcareous ring. Pelayothuria, therefore, is 

 probably a free-swimming form derived from an Elpidiid ancestor. 



FAMILY 4. CUCUMARIIDAE. Podia generally tube-feet only and no 

 papillae. Mouth and anus terminal or dorsal. Tentacles eight to thirty, 

 branched. Tentacle ampullae rudimentary or absent. Madreporite in- 

 ternal. Calcareous ring of five radial and five interradial pieces. Re- 

 tractor muscles. Longitudinal muscles generally simple. Respiratory 

 trees well developed. Cuvierian organs rare. Right and left tufts of 

 genital tubes. Spicules chiefly rods and knobbed, perforated plates. 

 Genera A. With distinct creeping sole. Ventral podia restricted to 

 radii Colochirus, Troschel ; Psolidium, Ludwig ; Theelia, Ludwig 

 (Fig. IV. 10) ; Psolus, Oken (Fig. IV. 8). B. Without distinct creeping 

 sole Thyone, Oken (Fig. IV. 11); Orcula, Troschel; Phyllophoras, Grube ; 

 all with scattered podia (generally). With podia more or less restricted 

 to the radii ; Cucumaria, Blainville (Fig. IV. 1, 2, 3, 4, and 5 ; Pseudo- 

 cucumis, Ludwig ; Actinocucumis, Ludwig; Echinocucumis,Snrs; Sphaero- 

 thuria, Ludwig (Fig. IV. 9). C. Flask-shaped, with mouth and anus close 

 together Rhopalodina, Gray (Fig. IV. 7). 



The Cucumariidae are distinguished by the possession of delicate re- 

 tractile arborescent tentacles (Fig. IV. 3 and 4). Frequently the two 

 median ventral tentacles are smaller than the others (Fig. IV. 3), as in species 



