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THE VOYAGE OF H.M.S. CHALLENGER. 
a rather thick integument, containing numerous small, irregular, reticulate 
cups and spheres, the latter barrel-shaped, sparsely provided with bars, so that the 
meshes become large; the former made up of a much closer network with smaller, 
more irregular meshes, and with the spinous opening closed by spinous bars. 
The sole seems mostly to have cups or rather flattened hollow bodies made up 
of an irregular network, and with spines in the outwardly directed surface. 
Only a few of the large dorsal scales have distinct pores, which possibly are in 
communication with the ambulacral system (?), and, if it be so, we have an 
example in a Psolus with dorsal papillae. The. retractor muscles are united to 
the longitudinal muscular bands by a muscular membrane iust as is described 
by Selenka in Psolus cataphractus. 
The following two species are very incompletely described and impossible to identify: — 
Psolus appendiculatus [Holothuria), Blainville, 1821 ; Jaeger, 1833, 
Habitat — Mauritius (Blainville, Hoffinan). 
Psolus forhesii, Couch, Peach, 1845. 
Habitat . — British Islands (Peach). 
The great conformity in internal and external organisation makes it impossible, for the 
present, to point out any true characters distinguishing the greater part of the 
species of this genus. It represents a very interesting group of Holothurians on 
account of the numerous transitional forms which unite the extremes. One 
may question whether all the forms characterised by the possession of only two 
lateral series of pedicels are anything else than varieties of one species. 
Subfamily 3. Sporadipoda 
Ambulacral appendages, almost without exception, in the shape of pedicels, scattered 
all over the body; seldom an arrangement of them in rows distinguishable. 
Tentacles ten to twenty. Calcareous ring of ten pieces, which are simple or 
compounded of smaller parts ; the five radial pieces often prolonged and bifurcate 
posteriorly. 
Genus 9. Thyone, Oken, 1815 ; Semper, 1868. 
Tentacles ten, two ventral smaller. Pedicels more or less crowded all over the body; 
very seldom an arrangement of them in rows along the ambulacra discernible. 
