36 OCEAN LIFE. 



another duty, and may be compared to fishing lines, ready to 

 seize upon such prey as comes within their reach and drag it to 

 the mouth to be devoured." 



HOLOTHUROIDEA. 



"A HOLOTHURIA may be regarded in one light as a soft Sea- 

 Urchin, in another as a radiated animal, approximating the An- 

 nalides." " 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 appearance, 

 is sufficiently striking. The integument which covers, or rather 

 forms the body, is entirely destitute of those calcareous pieces 

 which encase the Echini and Star-fishes ; but appears to consist 

 of a dense fibrous cutis of considerable thickness, covered exter- 

 nally with a thin epidermic layer. Beneath the cutis is another 

 tunic composed of strata of tendinous fibres crossing each other 

 in the midst of a tissue of a semi-cartilaginous 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. But although the calcareous shell of the Echinus 

 is thus totally lost, the locomotive suckers or feet already de- 

 scribed are still the principal agent employed in progression. In 

 many species, these organs are distributed over the whole sur- 

 face of the animal, and are protruded through countless minute 

 orifices which perforate the integument. In other cases, they 

 are arranged in five series, resembling the ambulacra of an 

 Echinus ; and in some instances they are only found upon the 

 middle of the ventral surface of the body, that forms a flattened 

 disc upon which the animal creeps somewhat in the manner of a 

 snail. The ambulacral feet themselves precisely resemble in all 

 the details of their structure those of the Asterias, and their pro- 



