234 THE HOLOTHURIOIDEA 



in the preceding family, the calcareous ring is well developed, often 

 bilaterally symmetrical, and with radial pieces strongly forked behind. 

 In Trochostoma and Ankyroderma the stone-canal is still connected with 

 the body -wall. Cuvierian organs have been described in Molpadia 

 chilensis, J. MiilL The calcareous spicules are very similar to those of 

 the Cucumariidae, except in Ankyroderma, which has peculiar anchor-like 

 spicules (Fig. I. 11), somewhat resembling those found in Synapia 

 (Fig. V. 6). Trochostoma has red calcareous deposits. 



The presence of retractor muscles, the structure of the calcareous 

 ring, and the general anatomy of the Molpadiidae indicate their close 

 relationship to the Cucumariidae. 



ORDER 2. Paractinopoda, Ludwig. 



The five radial canals have disappeared in the adult. There are no 

 tube-feet or papillae, and the tentacles are supplied directly from the 

 circular canal (Fig. V. 3). 



FAMILY, SYNAPTIDAE. Body cylindrical and elongated. Mouth and 

 anus terminal. Tentacles pennate or digitate. Tentacle ampullae small 

 or rudimentary. Calcareous ring of five radial, and frequently more 

 than five interradial, pieces. One or more internal madreporites. 

 Auditory vesicles on the radial nerves. Circular muscles uninterrupted 

 at the radii. Longitudinal muscles as a rule unpaired. Neither respira- 

 tory trees nor Cuvierian organs. Ciliated funnels on the coelomic epi- 

 thelium. Genital tubes in right and left tufts, often hermaphrodite. 

 Calcareous spicules as wheels, anchors, etc. Genera A. Spicules various ; 

 no wheels in the adult. Synapta, Eschscholtz (Fig. V. 1) ; Anapta, 

 Semper. B. With wheels Chiridota, Eschscholtz ; Trochodota, Ludwig ; 

 Trochoderma, Theel ; Myriotrochus, Steenstrup ; Acanthotrochus, Danielssen 

 & Koren. 



The tentacles of the Synaptidae vary in number from ten to twenty- 

 five. When they exceed ten in number there is usually a corresponding 

 increase in the number of the interradial pieces of the calcareous ring. 

 The radial pieces are often pierced anteriorly. There are some very 

 characteristic spicules, such as the many-spoked " wheels " (Fig. V. 7) 

 of some genera (and in the larva of Synapta), and the anchors and 

 anchor-plates in Synapta (Fig. V. 6, an, an.p). The perforated anchor- 

 plates lie in the cutis parallel with the surface, whilst the "shank" 

 of the anchors rest against them, the "arms" projecting towards the 

 surface. They aid locomotion % (6stergren, 1897). A pair of auditory 

 vesicles, or otocysts, has been found at the base of the five radial 

 nerves in Synapta and other forms (V. ox}. Sense organs, some pig- 

 mented and perhaps representing eyes, occur in some species on the 

 tentacles. The radial water-vascular canals, which are absent in the 

 adult, are temporarily developed in the larva (Fig. I. 8, r.c.c). The 

 tentacular canals, coming from the circular canal, may branch and 

 supply several tentacles. The layer of circular muscles is not inter- 

 rupted at the radii, as in the Actinopoda. Retractor muscles are de- 



