THE HOLOTHURIOIDEA 227 



ORDER l. Actinopoda, Ludwig. 



The five radial canals of the water-vascular system, springing from 

 the circular canal, supply the tentacles and podia (Fig. I. 5). 



FAMILY 1. HOLOTHURIIDAE. Body more or less flattened ventrally 

 to form a creeping sole. Mouth generally somewhat ventral in position. 

 Spicules chiefly in the form of tables, rods, and perforated plates. Cal- 

 careous ring of five radial and five interradial pieces. Ambulacral 

 appendages present in the shape of tube-feet, papillae, and peltate tentacles 

 (generally twenty). Kespiratory trees well developed. SUB -FAMILY 1. 

 SYNALLACTINAE. No tentacular ampullae. Stone-canal single, and joined 

 to the body-wall. No rete mirabile. Genera A. With genital tubes in 

 a right and left bundle Pseudostichopus, Theel ; Paelogatides, Theel ; 

 Meseres, Ludwig; Synallactes, Ludwig; Bathyplotes, Ostergren. B. 

 Genital tubes in a tuft on the left side only Mesothuria, Ludwig. 

 SUB-FAMILY 2. HOLOTHURIINAE. Well-developed tentacular ampullae. 

 Stone - canals often numerous, and with complex madreporites not con- 

 nected with the body-wall (exc. Holothuria lactea). The left respiratory 

 tree is enveloped in a vascular network, the rete mirabile. Cuvier's 

 organs often present. Genera A. With genital tubes in a tuft on the 

 left side only Muclleria, Jager, mostly tropical ; Labidorlema?, Selenka, 

 tropical ; Holothuria, Linn., to which genus belong the British Cotton- 

 spinner described above (p. 220), and H. edulis, one of the kinds of edible 

 Trepang. B. With genital tubes in a right and left tuft Stichopus, 

 Brandt. 



The Holothuriidae are characterised not only by the possession of well- 

 developed respiratory trees, and of from fifteen to thirty peltate tentacles 

 (formed of a thick stem branching at its extremity), but also by the 

 absence of retractor muscles, and of auditory vesicles. The tube-feet and 

 papillae may be in double radial rows, as in Labidodemas, in several rows, 

 as in Stichopus, or scattered, as in Holothuria. The calcareous ring is 

 generally composed of short, compact pieces ; it appears to be absent in 

 Paelopatides and Synallactes. The anus in Pseudostichopus is situated in 

 a deep vertical furrow, whilst in Muelleria it is surrounded by five 

 pointed, calcareous plates. The longitudinal muscles are in paired bands, 

 except in Pseudostichopus. 



The Holothuriinae are a well-differentiated group distinguished by 

 the possession of large tentacular ampullae projecting freely into the 

 body-cavity (Fig. II. 5, t.a), internal madreporites (5, md), and a vascular 

 plexus surrounding the left respiratory tree. The Synallactinae, on the 

 other hand, are in many respects intermediate between the former 

 sub -family and the Elpidiidae. The absence of tentacular ampullae, 

 and of the rete mirabile, and the presence of a single stone-canal con- 

 nected with the body- wall and probably retaining its primitive opening 

 to the exterior, are all characters uniting the Synallactinae to the 

 Elpidiidae, with which they have indeed been classified (Ostergren). 



FAMILY 2. ELPIDIIDAE. Body generally flattened ventrally. Mouth 

 more or less ventral. Tentacles, ten to twenty, more or less peltate. Ventral 



