100 CLASSIFICATION OF THE ANIMAL KINGDOM. 



opens into a circular arterial tube that surrounds the mouth: the object 

 of this organ is quite unknown, but it is used probably for the purposes 

 of respiration. This latter function is likewise exercised by the admission 

 of sea-water into the interior of the system, which bathes the entire inner 

 surface of the viscera; and this is further carried out by the action of 

 vibratile cilia, which are extensively distributed over the external and 

 internal surfaces of the body. The organs of reproduction are of the 

 simplest structure, there being no distinction of sex; and the nervous 

 apparatus, which is filamentary and not ganglionic, is well defined. 

 Fhrenberg thinks that he has discovered eyes in some species of Star- 

 Fish, but apparently without good foundation. They can appreciate the 

 most delicate touch, and have the power of casting off their rays when 

 alarmed, which often sprout out again. 



With regard to the Echinidce, the outward shape of the common type 

 is well known to all. It consists of a hard shell, composed of innumerable 

 small plates (some of which are perforated) accurately joined together, and 

 covered with spines and tubercles, which are used for locomotive purposes; 

 and the whole external surface of this shell when alive is covered over 

 with a thin vascular membrane. Through the perforated plates the auiinal 

 can extend a great number of tubular feet or suckers, exactly analogous 

 to those of the Aateridce. The interior of the animal is most wonderful, 

 the mouth is a simple orifice armed with jaws, which are worked by an 

 elaborate set of muscles, and five sharp teeth. "These jaws," says the 

 author, "from their great complexity and unique structure, form perhaps 

 the most admirable masticating apparatus met with in the whole animal 

 kingdom." They also possess an oesophagus and stomach, without intestinal 

 division. Their circulatory system is very extensive, having a large intestinal 

 vein with numerous arteries. Eespiration is effected by the same means 

 as that used by the Star-Fishes, namely, by the copious admission of the 

 surrounding element into the interior of the body; and besides this, according 

 to Delle Chiaje, they possess a series of tentacula in the neighbourhood of 

 the mouth, which are capable of performing the office of branchw. Little 

 is known about their nervous system, and, like the Star-Fishes, they have 

 no distinction of sex. 



The Holothuridue, or Sea Cucumbers, are closely allied to this last family, 

 but instead of a calcareous covering, they possess a dense fibrous cutis of 

 considerable thickness, covered externally with a thin epidermic layer. 

 Their muscular system is well developed, and, like the Echini, they possess 

 suckers or feet, distributed either serially or all over the surface of the 

 body, which they use as instruments of locomotion. Around their mouths 

 is a circle of retractile tentacula, and the structure of their digestive apparatus 

 is similar to that found in the Echinus, the teeth of which last mentioned 



