172 ECHINODERMATA. 



however, owing to the complexity of the dental apparatus, has not 

 been satisfactorily demonstrated, although analogy would lead us 

 to infer the existence of such an arrangement. 



(21 2.) The Echini, like the star-fishes, exhibit no distinctions of 

 sex : all are fertile, and in the structure of their reproductive organs, 

 display, if possible, greater simplicity of arrangement than even the 

 Asteridce above described. The ovaria are five delicate mem- 

 branous bags, quite distinct from each other, which open exter- 

 nally by as many delicate tubes, or oviducts, as we may term them. 

 The apertures through which the eggs escape are easily seen upon 

 the outer surface of the shell, placed around the anus ; and are re- 

 cognisable not merely by their size, but from the circumstance of 

 each perforation being placed in the middle of a distinct oval 

 plate of the shell, distinguished by zoological writers as the ova- 

 rian pieces. The membranous sacs in which the ova are secreted 

 vary in size, in proportion to the maturity of the eggs contained 

 within them, and at certain times of the year are enormously dis- 

 tended : it is in this state that the " roe of the sea-egg," as the 

 ovaria are commonly called, is used as an article of food ; and in 

 some countries, especially upon the shores of the Mediterranean, 

 they are eagerly sought after, when in season, by divers employed 

 to procure them. 



(213.) Holothuridce. The name applied by naturalists to the 

 animals composing the next family of Echinodermata is derived 

 from a Greek word of uncertain application (oAo0oup*ov). In 

 common language they are generally known by the appellation 

 of " sea-cucumbers ;" and in fact, to a casual observer, the resem- 

 blance which they bear to those productions of the vegetable 

 kingdom, both in shape and general appearance, is sufficiently 

 striking. The surface of these animals is kept moist by a 

 mucus, which 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 

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

 dense fibrous cutis of considerable thickness, covered externally 

 with a thin epidermic layer. Beneath the cutis is another tunic com- 

 posed of strata of tendinous fibres crossing each other in the midst of 

 a tissue of a semicartilaginous nature, which is capable of very great 

 distension and contraction, and serves by its elasticity to retain the 

 shape of the body. Within this dense covering are seen muscular 



