202 ECH1NODEEMATA. 



plate of the timepiece to which, as to its shape, the creature has been 

 already compared. The round shield-like plate thus formed is divided 

 by a cinquefoil-shaped figure into five compartments, and constitutes 

 the first rudiment of the future Echinus ; as its size increases, new 

 divisions make their appearance upon its periphery, indicating the situ- 

 ations of the future tentacles or feet ; and soon afterwards little round 

 tubercles begin to develope themselves, which gradually rise up into 

 cylindrical elevations and ultimately assume the appearance and texture 

 of the locomotive spines. 



(530.) The shield itself, forming the basis upon which the apparatus 

 of suckers and spines is supported, is now seen to enclose in its sub- 

 stance its own proper calcareous skeleton : this consists at first of minute 

 detached triradiate spicula, w r hich, as they increase in number, arrange 

 themselves so as to constitute a sort of network in the texture of the 

 skin, wherein ultimately the polygonal calcareous plates of the shell 

 make their appearance. 



(531.) HOLOTHUHIDJS. The name applied by naturalists to the ani- 

 mals composing the next family of Echinoderinata is derived from a 

 Greek word of uncertain application (6\o6ovpiov). In common language 

 they are generally known by the appellation of " Sea-cucumbers "; and 

 in fact, to a casual observer, the resemblance which they bear to those 

 productions of the vegetable kingdom, both in shape and general appear- 

 ance, is sufficiently striking. The surface of these animals is kept moist 

 by a mucus that continually exudes through innumerable pores and 

 appears to be secreted by minute follicles imbedded in the substance of 

 the skin. The integument which covers, or, rather, forms the body, is 

 entirely destitute of those calcareous pieces that encase the Echini and 

 Star-fishes ; it appears to consist of a dense fibrous cutis of consider- 

 able thickness, covered externally with a thin epidermic layer. Be- 

 neath the cutis is another tunic, composed of strata of tendinous fibres 

 crossing each other in the midst of a tissue of a semicartilaginous 

 nature, which is capable of very great distention and contraction, and 

 serves by its elasticity to retain the shape of the body. Within this 

 dense covering are seen muscular bands running in different directions, 

 which by their contraction give rise to the various movements of the 

 creature : of these muscles five strong fasciculi assume a longitudinal 

 course, passing along the entire length of the animal from the mouth 

 to the cloaca; and in the interspaces between these, circular and oblique 

 muscles are readily distinguishable. The whole of this muscular case 

 is lined with a delicate membrane or peritoneum, from which processes 

 pass inwards to support the various viscera. 



(532.) But although the calcareous shell of the Echinus is thus totally 

 lost, the locomotive suckers or feet already described are still the prin- 

 cipal agents employed in progression. In many species, as in that 

 represented in the annexed figure (fig. 100), these organs are distributed 



