VI. Holothurioidea. 19 



VI. Holothurioidea. 



See also Bury, supra, p 4, Cuenot, supra, p 6, Hamann, supra, p 4, Kor- 

 schelt( 1 ), supra, p 3, Mac Munn( 1 , 2 ), supra, p 6, Semon( 1 ), supra, p 5, Sluiter. 

 supra, p 8. 



Ludwig s ( 2 ) detailed exposition of the morphology of Holothurians contains 

 some new points of interest. In 3 tropical species of Synapta (grisea, kefersteinii, 

 and serpentina] the anchors appear before the anchor plates, and do not pierce 

 the latter, but merely rest on them, as stated by Semper. When the plates of the 

 armature occur in different layers of the integument, those near the surface are 

 generally smaller and more spinous than the plates lying deeper. Excepting the 

 peculiar coloured plates of Trochostoma and Ankyrodenna, all the various types 

 of the armature may be derived from the common fundamental form of a rod with 

 tendency to repeated forking at both ends. The stools of the Aspidochirotae are 

 homologous with the four armed crosses of the Elasipoda, which are persistent 

 young stages of the primary stool plates ; but the lowest cross piece of the latter 

 has a different morphological value, according as the plate is flat or convex. - 

 L. adopts the views of Semper and Semon respecting the structure of the radial 

 nerve, regarding it as composed of a thicker external (u 1 ) and a thinner internal 

 layer (n 2 ), which are separated by a cross partition. The nerve ring is formed 

 from n 2 only, but n 2 is probably also nervous, as it sends fibres to the tube-feet. 

 When only one Polian vesicle is present, it is in either the left dorsal or left 

 ventral interradius. The Molpadiidae and among the Elasipoda the Deimatidae 

 and Psychroprotidae are the only families with one ; then follow in order of com- 

 plexity, the remaining Elasipoda, Aspidochirotae, Deudrochirotae, and Synapti- 

 dae. The tentacular canals of all Holothurians except the Synaptidae are 

 lateral branches from the enlarged basal portions of the radial water-vessels, and 

 do not arise directly from the oral ring, as was first shown by The'el in the Elasi- 

 poda. Hamann's observations indicate that this is also the case with 5 of the 12 

 tentacular canals in Synapta digitata. The tentacular ampullae are well developed 

 in Aspidochirotae, smaller in Molpadiidae and especially in Synaptidae, while 

 there are none in Elasipoda and Dendrochirotae [see Herouard, infra, p 20]. - 

 The Molpadiidae and Elasipoda have but one water- tube ; Synaptidae, Dendro- 

 chirotae , and Aspidochirotae follow in ascending order , the number reaching 

 (JO-80 in Holothuria chilensis. Where many water-tubes are present, the Polian 

 vesicles also increase in number. The right side of the body generally has more 

 water-tubes than the left, and when there is only one, it is dorsal, and usually 

 median, with more tendency to be displaced to the right than to the left. - - In 

 the Molpadiidae, Elasipoda and Dendrochirotae the longitudinal muscular fibres 

 of the gut are always inside the circular ones. Some species of Synapta and Chi- 

 ridota rufescens have this order inverted ; while in the Aspidochirotae the arrange- 

 ment is usually normal in gullet and cloaca, but inverted in the mid-gut. In all 

 Holothurians there are two loops in the gut, but they are so very open in the 

 Synaptidae that it appears almost straight. Between mouth and anus the gut also 

 makes a spiral twist through the body, following the watch-hand, as seen from 

 the oral pole. It commences in the mid-dorsal interradius and terminates in the 

 right ventral one. This may be traced by the line of attachment of the mesentery 

 which falls into 3 parts, dorsal, left, and right, corresponding to the 3 divisions 

 of the gut. They are attached respectively along the median lines of the mid- 

 dorsal, left dorsal, and right ventral interradii. The Synaptidae and Deimati- 

 dae are the only Holothurians entirely without internal gills (so-called lungs) ; 

 for they seem to be represented in Elpidiidaeand Psychroprotidae by a diverticulum 



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