406 



ECHINOZOA. 



face of the arms have the peculiarity that each is constricted in 

 the middle, a feature which would rather lead to the belief that 

 these plates are truly of the nature of "ambulacral ossicles." 



Class IV. Holothuroidea. 



The class of the Holothuroidea includes the animals usually 

 known as " Sea-cucumbers," distinguished from the other Echino- 

 derms by their elongated, vermiform, or slug-like form, and their 

 leathery muscular integuments. The mouth and anus are usually 

 terminal in position, and the radial symmetry of the body is not 

 conspicuously shown externally except by the crown of oral ten- 

 tacles, and, often, by the bands of tube-feet. Palaeontologically 

 speaking, the Holothurians are of little importance, since they 

 possess few structures which are capable of preservation in a fossil 

 condition. Unlike the other Echinoderms, the Holothurians ex- 

 hibit a very limited tendency to a calcification of their tissues. No 

 proper "test," in fact, exists in any Holothurian, but the skin con- 

 tains numerous isolated, mostly microscopic bodies, of special 

 forms in different types. These calcareous structures (fig. 283) 

 may be globular, wheel-shaped, spicular, anchor-shaped, &c, and 



Fig. 283. — Integumentary ossicles of recent and fossil Holothurians. a, A plate of Ackistrum 

 Nicholsoni, enlarged forty-five times, from the Carboniferous rocks of Scotland ; b, Hooklet of 

 the same, similarly enlarged ; c, Anchor and anchor-plate of a recent species of Synafita, en- 

 larged ; d, Plate of Chlrodota ? Traquairii, enlarged forty-five times, from the Carboniferous 

 rocks of Scotland ; e, Plate of the recent Thyonidium pellucidum, enlarged. (After R. Etheridge, 

 jun.) 



in rare cases (as in Psolus) have the form of comparatively large 

 imbricated scales, which constitute a kind of external shell. When- 

 ever the integumentary calcifications are plate-like, they exhibit 

 under the microscope the peculiar netted structure which is char- 

 acteristic of the Echinodermal test. In addition to the integument- 

 ary hard structures, there exists a ring of calcareous pieces sur- 

 rounding the mouth, and serving for the attachment of five great 

 longitudinal muscles. 



Owing to their want of hard structures of any size, the geological 

 history of the Holothurians is necessarily a very imperfect one, being 



