296 EVENINGS AT THE MICROSCOPE. 



tures we have been looking at, have still the lengthened 

 form, which, however, so closely resembles that of a warty 

 angled cucumber that the animals I allude to are familiarly 

 called Sea-cucumbers (Holothuriadce). The marine zoo- 

 logist frequently finds them beneath stones at extreme low 

 water ; and larger forms, as big in every direction as a 

 marketable cucumber, are occasionally scraped from the 

 bottom of the deep sea by means of that useful instrument, 

 the dredge. If you drop one of them into sea-water, you 

 will presently see from one extremity an exquisite array 

 unfold, like a beautifully cut flower of many petals, or, 

 rather, a star of ramifying plumes. Soon it begins to 

 climb the walls of your aquarium, and then you catch the 

 first glimpse of its affinity to the Urchins ; for the short 

 warts which run in longitudinal lines down the body 

 corresponding to the angles, gradually lengthen them- 

 selves, and are soon perceived to be sucking-feet, analogous 

 in structure and in function to those with which the 

 Star-fish and the Sea-Urchin creep along. 



But the relationship becomes more apparent still when 

 we find that the Cucumber has a skeleton of calcareous 

 substance deposited on exactly the same plan as in the 

 Urchin, viz., around insulated rounded cavities. It is true 

 you may cut open the animal and find nothing at all more 

 solid than the somewhat tough and leathery skin ; but a 

 calcareous skeleton is there notwithstanding, though in 

 truth only a rudimentary one. If we were to cut off a 

 considerable fragment of the skin, and spread it out to dry 

 upon a plate of glass, and then cover it with Canada 

 balsam, we should find (assisted by the translucency 

 which is communicated to the tissues by the balsam) that 

 the skin is filled with scattered atoms of the calcareous 

 structure, perfectly agreeing with that with which the 

 solid framework of the Urchin is built up ; but minute 

 and isolated in the flesh, instead of being united into 

 one or more masses of definite organic form. 



