3ri2 



EcHINOIDEA.- 



-RADIATA.- 



-ECHINID.E. 



Okdeb I.— ECHINOIDEA {Sea-e</r/s, or Sca- 

 urcliiiis.) 



The animals belonging to this order (see Plates 1-4, 

 Echinodermata) are covered with a round or oval hard 

 case, composed of perpendicular bands of square or 

 six-sided pieces sunk in the substance of the skin. 

 Externally tliis case is studded all over with numerous 

 spines which are affixed on hemispherical tubercles by 

 particular muscles, and easily fall off when the animal 

 is dead (see fig, annexed). These spines are various 

 in shape and size, move in all directions; and — ^judg- 

 ing on the principle that Nature does nothing in vain, 

 and that even the apparently insignificant peculiarities 

 of form which she has given to creatures are designed 

 to serve some important end connected with their 

 wellboing — appear to be useful in protecting the ani- 

 mals from their enemies, and enabling them also to bury 

 themselves in tlie sand on the shores wlien they are left 

 by the retiring tide. Half the number of the perpen- 

 dicular bands of which the case is composed, alternating 

 with the others, are pierced with minute double pores 

 ill rough which are sent out small filaments with dilated 



Fig. 234. 



ends, that enable the animals to anchor themselves to 

 marine bodies. These pierced bands, from a fancied 

 resemblance to the walks in a garden, are called 

 ambulacra ; while the alternate ones which have the 

 tubercles have been compared to the beds of the gar- 

 den, and have in consequence been sometimes called 

 nrece pulvilli (see Plate 1, fig. 4). These animals 

 have two separate openings to their digestive canal, 

 and in some of the species the vent is placed in 

 the centre of the lower surface, whilst in others it is 

 situated on the side of the body. The mouth is armed 

 wiih very complicated jaws, and furnished with five 



projecting teeth (see Plate 1, fig. 3 ; Plate 3, figs. 5, G). 

 These jaws were well described by the ancient natural- 

 ists, and are known by the name of the " lanterns of 

 Aristotle," that author having represented them as 

 resembling a lantern. The animals belonging to this 

 order are carnivorous, living chiefly upon small niol- 

 lusks and Crustacea. The recent species are numerous, 

 and are found in most parts of the world ; while the 

 fossil remains of their shells or external cases are 

 abundant in many of the formations, but especially in 

 the chalk. 



The numerous species of which the order is com- 

 posed, may be divided into six families : — 1, Echinidw; 

 2, Cidaridm; 3, ScutelUdce ; 4, G alcritida: ; 5, Echi- 

 nolampasidm ; and 6, Spatangidm. 



Family L— ECHINID^E {True Sea-eggs). 



This family is perhaps the most numerous in species, 

 contains some of the best known and most commonly 

 distributed, and may be taken as the most typical of 

 all. The shell or case is generally rounded and com- 

 posed of twenty perpendicular bands of square or six- 

 sided pieces. The tubercles upon which the spines 

 are placed are simple, rounded, im- 

 perforate prominences. The spines 

 vary a good deal in shape. Gener- 

 ally, as in Echinus, they are equal- 

 sized, conical and striated. In 

 others, as in Colohocentrus, they are 

 large and club-shaped ; while in a 

 third set, as in Ilelerocentriis, they 

 are very short and truncated, form- 

 ing in the mass a smooth surface 

 — somewhat resembling a tesse- 

 lated pavement. The ambulacra 

 are constantly of the number of five 

 pairs (see Plate 2, fig. 3), and aie 

 complete, extending from the upper 

 to the lower surface. In addition 

 to the spines on the tubercles and 

 the filaments which are sent oiit 

 from the ambulacral pores, and 

 which serve as organs of pro- 

 gression, the Echini possess other 

 movable organs scattered over the 

 surface of the body, and known by 

 the name of Pedieellariai. These 

 organs though small, are very curi- 

 ous, and perform a very important 

 part in the economy of these ani- 

 mals. They are prehensile, and are used by the animal 

 to seize hold of objects, which can thus be passed 

 from one to another, and possibly may even be con- 

 veyed from the dorsal surface to the mouth. They 

 are very numerous, crowded irregularly on most parts 

 of the skin, but especially round the oral aperture. 

 They are of various forms, but in general they may be 

 described as having a thick head, cleft into three 

 divisions, and set on a long, slender, flexible stalk. 

 The whole body and head are invested with a glutin- 

 ous flesh, in which are imbedded minute red glands, 

 that are common to the integument of the whole class, 



