544 THE ANIMAL KINGDOM. 



The typical Echinoderm larva is a bilaterally symmetrical organism 

 with well-marked dorsal and ventral surfaces, the latter the smaller of 

 the two. The mouth is ventral and subterminal at one pole, the anus 

 (= gastrula mouth) ventral and subterminal at the other, and the 

 digestive tract takes an antero-posterior course between the two with 

 a curve convex to the dorsal surface. The right and left peritoneal 

 (= coelomic) sacs unite around it, and the dorsal union between the 

 two walls persists as the mesentery, at the anterior end of which is the 

 water-tube and pore. The Holothurioidea retain some to a greater, others 

 to a less extent these characteristics. Their nervous and water-vascular 

 systems with the longitudinal muscle bands are, however, invariably 

 disposed radially. In all other Echinoderms there is a paramount radial 

 and with few exceptions pentamerous symmetry, and the only trace of 

 the primitive bilateral symmetry is to be found in the mesentery support- 

 ing the water-tube and plexiform organ ; but though this mesentery is 

 more or less antero-posterior it does not lie in the primitive median 

 dorso-ventral plane. A secondary plane of bilateral symmetry however 

 is often established, as in irregular Echinoidea and the Crinoidea. Further- 

 more the dorsal or abactinal surface of the adult corresponds to the 

 right side with more or less of the dorsum of the larva, and the ventral 

 or actinal surface to its left side with more or less of the ventral surface. 

 The mouth of the adult is in the centre of the actinal surface, the anus 

 excentric on the abactinal, though in the course of growth it may travel 

 round interradially to the oral surface. It must be carefully noted, there- 

 fore, that the point from which nerves and water-vascular vessels radiate 

 in the Holothurioidea, lies in the larval antero-posterior axis, and that 

 they are parallel both to this axis and the median dorso-ventral plane. 

 But in other Echinoderms the point from which they radiate lies on the 

 left side in a dorso-ventral axis at right angles approximately both to 

 the primitive antero-posterior axis and median dorso-ventral plane. 



The calcareous deposits in the integument take the form of spicules 

 or variously shaped plates. The Holothurioidea alone retain a muscular 

 integument. In all other Echinoderms the muscular system is greatly 

 aborted and is only developed in relation with the movements of special 

 parts of their skeleton. In this skeleton there are two important systems 

 of plates, the apical and oral, corresponding respectively to the centres 

 of the dorsal and oral surfaces in the adult. The apical system comprises 

 a dorso-central plate round which are arranged, passing from the centre 

 outwards, at first in a spiral but afterwards in a circle, (i) five basal plates 

 interradial in position ; (2) five radial plates radial in position. To these 

 may be added (i) within 'the circle of basals, a circle of five underbasal 

 plates radially placed, and (2) externally to the radials five primary inter- 

 radial plates. Underbasals occur only in Asteroidea, Ophiuroidea and 



