50 



NA TURA L HIS TOR Y. 



worm-like ; the mouth is without teeth, and the body usually lacks the 

 hardened envelope so characteristic of the starfish and sea-urchins. The 

 common name, " sea-cucumbers," is very descriptive of the typical forms as 

 they appear when brought to the surface. The general shape of the body 

 is the same, and the resemblance is strengthened by the fact that the tube- 

 feet of the marine animal closely simulate the spines on the surface of the 

 familiar vegetable ; the color, however, is usually greatly different in the 

 two, shades of green being far from common in the sea-cucumbers. 



In their native element the sea-cucumbers show many features which 

 are not seen in the specimens brought to the air. Oar plate shows three 

 individuals, two of which have the crown of tentacles surrounding the 

 mouth widely expanded, while the tube-feet along the sides of the body are 

 distended to their fullest extent. 



While the sea-cucumbers (except in forms like Psolus) lack a hardened 

 investment of the body, still all show traces of such a protection, for in 

 the skin are numerous plates of carbonate of lime. Usually these plates 

 form, an irregular network, perforated by innumerable holes, and scat- 

 tered irregularly through the skin; but in some species they are very 

 regular in their shape, and are among the favorite show specimens of the 



microscopist. Thus in a few genera these 

 plates, microscopic in size, take the shape 

 of wheels, with their hub, spokes, and 

 rim. One of these wheels from a northern 

 species is figured here. In another genus 

 (Synaptd) the plates are very peculiar. 

 They occur in pairs, each pair consisting 

 of a flat, fenestrated plate and a calca- 

 reous anchor, which in shape is almost 

 exactly like the large anchors used in 

 mooring vessels. 



One species of Synapta closely like that 



Fi.;. 4S. — One of the "wheels'* embedded in sn0W n ill OUT figure is Common Oil OUT 

 the skin of Myriotrochus. ° 



Atlantic coast. It forms slender burrows 

 in the sandy shore, near low-water mark, and it is said that the anchors 

 covering the skin are of use in holding the animal in place in the tube. 

 The animal is transparent, and when taken from its burrow is a very 

 attractive object. Placed in a dish of water it soon expands its tentacles, 

 and empties its digestive canal of sand, and then almost every detail of 

 structure can be seen. The cut shows one of these forms, in which it will 

 be noticed that the body is constricted at intervals. These constrictions 

 are caused by the contraction of the circular muscles, and may take place 



