HOLOTHURIOIDEA. 553 



two (rarely more) branched outgrowths the respiratory trees homologous 

 with the two interradial intestinal caeca of Asteroidea. The mesentery 

 in the Apneumona bears, especially at its base, a number of ciliated funnel- 

 shaped bodies either scattered or in groups. Processes known as Cuvierian 

 organs are attached to the cloaca. They are invariably absent in Synap- 

 tidae, and perhaps also in some others. They are discharged when the 

 animal is irritated, and have been variously stated to be solid or to contain 

 an axial cavity. 



The generative organs consist of either a single bundle of caeca at- 

 tached to the left side of the mesentery anteriorly, or of two bundles right 

 and left (Stichopus among Aspidochirotae, Dendrockirotae, and many 

 Elasipoda). The duct is single and opens by a single pore placed in the 

 dorsal median line outside the circle of tentacles or within it in Dendrochi- 

 rotae. In some Elasipoda, however, it branches and opens by a cor- 

 responding number of pores. The sexes are separate with the exception 

 of the Apoda Apneumona and Pneumonophora (?). 



Psolinus developes without a metamorphosis, and the embyro is not 

 ciliated. Cucumaria passes at once into the pupa stage, while others, so 

 far as observed, pass through an Auricularia into a pupa stage; see p. 548. 

 The larval mouth and anus persist in the adult. 



Holothurioidea are found in all seas. The Aspidochirotae swallow 

 quantities of sands, &c. ; others feed on small animals. Fossil wheels (of 

 a Chirodota^) have been found in Jurassic strata, and remains of a Chiro- 

 dota and Synapta even as early as the Carboniferous strata in Scotland. 



The Holothurioidea may be divided into 



1. Elasipoda : primitive deep sea forms, with a well-marked bilateral symmetry, 

 tube feet restricted to the flat ventral surface, and papillae on the dorsum. The 

 stone-canal often opens externally by a pore, and there are no respiratory trees. 

 (Elpidiidae, Deimatidae, Psychroprotidae). 



2. Pedata, with well-developed tube feet and papillae, subdivided into 



(a) Aspidochirotae, with peltate tentacles provided with ampullae, with five 

 radial and five interradial pharyngeal ossicles, and left respiratory tree loosely con- 

 nected to the mesentery ; e. g. Holothuria, Stichopus. 



(f) Dendrochirotae : with arborescent tentacles, retractor muscles to the 

 pharynx, and two sets of generative caeca ; e. g. Cucumaria, Psolus. 



3. Apoda, devoid of tube feet and papillae, subdivided into 



(a) Pneumonophora ( Molpadidae\ with the left respiratory tree connected 

 as in 2. a ; ? hermaphrodite. 



() Apneumona (= Synaptidae} : devoid of radial water- vascular vessels, 

 respiratory trees, and Cuvierian organs ; hermaphrodite ; e. g. Synapta, Chirodota. 



Holothurioidea, Theel, Challenger Reports, xiv. 1886. Elasipoda, Id. ibid. iv. 

 1882. Variations in Holothurians, Lampert, Biol. Centralblatt, v. 1885-6. 



Histology, Hamann, Beitrage, &c., 'Die Holothurien,' Jena, 1884; Id. Z. W. Z. 

 xxxix. 1883; Jourdain, Annales Mus. Nat. Hist. Marseilles, i. 1883. 



