CLASS E C H I N O D E R M ATA. 



ORDER E CHINOIDA. SEA-URCHINS. 



149 



reproduction, if the division be made near the head, or close to the extremity 

 of the tail ; for the body, remaining alive, renews 

 either of those extremities. But if the animal be 

 severed in the middle, life becomes extinct in a few 

 hours. 



The Tubicolous Annelids are represented by the 

 genus Serpula, the generic characters of which are 

 Animal inhabiting a solid, calcareous tube, more or 

 less irregularly twisted and fixed upon some extra- 

 neous substance ; its body elongated, slightly de- 

 pressed, and attenuated behind, composed of nu- 

 merous narrow segments, armed on each side with 

 a row of subulate and hooked bristles ; branchiae 

 terminal, fan-shaped, divided into numerous plu- 

 mose digitations ; mouth terminal, situated between 

 the branchiae, and surrounded by a pedicellate funnel 

 Group of Serpula. or club-shaped operculum. 



Type of the genus, <S. wrmcularis (Linnaeus). Several species occur, 

 both recent and fossil ; of the former, five or six are found on the coast of 

 Britain. 



The En-antes are formed for locomotion, thus presenting a striking con- 

 trast to the Tubicolous tribe. The genera are found to vary in size from 



Xereis. 



one or two inches to fifteen or sixteen feet. Some of the Errantes are 

 common around the coasts of England and Ireland. The Nerds belong to 

 this division. 



CLASS XI.-ECHINODERMATA. 



THE tubular feet with which the Star-fish, Sea-urchins, and Holothurise are furnished, led Cuvier to place them together in his Pedi- 

 cellate Order ; but their form and internal structure vary so considerably, that they are justly entitled to be ranged into an inde- 

 pendent Class, having three distinct Orders. 



ORDER I. ECHINOIDA. SEA-URCHINS. 



THE Ecliinoid Order (printed Echidna by mistake on Plates 1 and 2). 

 This division is generally known by the name of Sea-urchins, or Sea-eggs, 

 their exterior consisting of a calcareous shell, which in some, as the Echini, 

 has a flattened spheroidal shape, its mouth, armed with five strong teeth, 

 being below, and the vent above. In others, as Galerites, it has a conical 

 form with swelling sides, the mouth is central in the base, and the vent at 

 its edge ; and in some, as the Spatangi, its inferior surface, of an oval shape, 

 flat or slightly hollowed, has towards its front margin a transverse toothless 

 mouth, and the vent near its hind edge, whilst its upper surface is more or 

 less convex. 



Character of the group. Body not contractile, nor radiately lobed, mostly 

 globular ; skin hard, furnished with tubercles provided with mobile spines ; 

 digestive canal having a mouth and vent. 



The shell of these animals is composed of numerous regularly-disposed 

 plates, united on all sides by a straight suture, and furnished externally with 

 rounded tubercles, on which the mobile spines are attached. The pieces 

 form twenty perpendicular bands, each composed of several horizontal penta- 

 gonal pieces ; the bands are placed symmetrically in pairs, uniting together 

 by a flexuous suture ; the projecting angles of one series being fitted into 

 the concave angles of the other. The pairs of bands are united together 

 generally by a straight suture, and the pairs are alternately broad and narrow. 

 The broad 1 ands are formed of a few pieces, and are always imperforated, 

 and the outer edge of the narrow bands, which consist of very numerous 

 long narrow pieces, are perforated by two or more series of minute holes 

 placed together in pairs ; these perforations form grooves in the covering of 

 the body, which Linnaeus compared to the walks in a garden, and called 

 Ambulacra ; and he distinguished the parts which are covered with tubercles 

 by the name of Arete pulvilli or beds. 



The spines are attached to the base of the tubercles by a circular ligament, 

 lined with muscular fibres moving them in every direction. 



The plates are placed between the two skins with which the body of 

 these animals are covered ; the outer of these skins is the thickest : it is 

 greatly thickened near the vent and mouth ; and the plates, which are com- 

 posed of fibres perpendicular to their surface, and increase in size by the 

 addition of particles to the inner side of the fibres, and by the formation of 

 new ones on the sides of the older, are kept separate from each other by a 



small process of the skin being interposed between them, so that they never 

 become soldered together during the life of the animal. 



The spines evidently grow by a deposition of matter placed on their outer 

 edge, more especially near the distal extremity ; the matter is perhaps depo- 

 sited by the processes of the outer skin, which forms the articulation, being 

 extended up the longitudinal grooves with which these spines are furnished. 



The vent is surrounded by numerous small scale-like pieces attached to 

 the skin, which are generally regularly disposed, but vary in the different 

 families. In the typical families, where the vent is placed on the centre of 

 the back, just opposite to the mouth, it is surrounded by two series, each 

 formed of five pieces, which are attached to the body of the crustaceous 

 covering : the series of these plates which are next the body are the smallest ; 

 they are placed just at the top of the ambulacra, and each is perforated with 

 a minute hole, the use of which is quite unknown. 



The mouth varies in the various families of the group in the Spatangidce ; 

 it is destitute of any hard parts, but is furnished with tentacula, compared 

 by some to the similar parts in the Holothurioe. In the ScuteUidoe it is fur- 

 nished with five triangular cellular bones, each provided with a blunt, arched 

 tooth at the inner angle, which apparently serves for crushing the food ; 

 while in the typical families this part is furnished with a much more compli- 

 cated apparatus, with short prominent teeth fit for biting their food. These 

 jaws were compared by Aristotle to a lantern ; they consist of ten conical 

 triangular bones, soldered together in pairs, containing between them a long 

 linear curved tooth. The teeth are externally convex, and furnished with 

 an internal central rib, and the end hardens as they are worn away by use. 

 These jaws are articulated together by the intervention of oblong bones 

 converging towards the centre, and furnished with five other linear arched 

 bones. The jaws are moved by muscles placed between them, and by otheis 

 attached to five variously-formed erect processes, placed on die oval edge of 

 the body of the crust, called auricuks by Blainville. 



Breynius, in 1732, divided the Echini into seven genera, from the 

 position of their mouth and vent ; which arrangement Lamarck adopted, 

 but under other names. Klein subsequently divided them into nine sections, 

 containing twenty-two genera, of which he fonned two systems. Leske 

 published, in 1778, an addition to Klein ; in which he considered Klein's 

 sections as genera, and adopted the prior names given by Breynius. To 

 the genera of Breynius, Lamarck, who wrote from actual examination of 

 the class, has added a new genus, which he names Cassidtdus. 



