220 THE HOLOTHURIOIDEA 



structure, development, and taxonomy of the Holothurioidea has 

 recently been written by the same author (5), to whose works the 

 present account is greatly indebted. 



As a typical example of the Holothurioidea the common Cotton 

 spinner, Holothuria forskali, Delle Chiaje (H. nigra, auctt.), may be 

 taken. It is about 20 cm. long, almost cylindrical in shape (Fig. I. 2), 

 pale yellowish -brown beneath, and black above. The "ventral" 

 or lower surface is covered with closely set retractile podia, by 

 means of which the animal creeps along (2, p) ; the lateral and 

 " dorsal " surfaces are covered with small and large conical papillae 

 (2, pp). The five "radii" running longitudinally from mouth 

 to anus are scarcely distinguishable on the outer surface; there 

 are two dorsal radii (bivium), and one median ventral, and two 

 lateral ventral radii (trivium). Near the anterior end the body- 

 wall is produced into an irregular brim (2, br) ; beyond this the 

 animal terminates in a smooth pale-coloured area, in the centre of 

 which lies the mouth, somewhat ventral in position (2 and 3, m). 

 Surrounding the mouth is a set of twenty semi-retractile tentacles 

 (2 and 3, /). When in a state of contraction the brim closes 

 over the retracted tentacles and mouth. The anus is situated at 

 the posterior pole of the animal. 



The body -wall of Holothuria is thick, tough, and leathery. 

 Externally a thin transparent cuticle covers the epidermis, which is 

 not ciliated, and is composed of a layer of columnar cells, with 

 scattered gland cells and sensory cells. The thick underlying cutis 

 is formed of connective tissue cells and numerous fibres lying in a 

 homogeneous ground substance. "Wandering cells are found in the 

 cutis, while the greater part of the pigment is in its outermost layer. 

 Below the cutis is the layer of circular muscles (Fig. II. 5, c.m) l 

 interrupted at the radii, except immediately round the anus, where 

 it forms a sphincter muscle. A paired band of longitudinal muscles 

 runs along each radius from the posterior end of the animal to 

 the anterior (Fig. II. 1 & 5, Lin) ; here it is attached to the radial 

 plate of the calcareous ring to be described below. Internally the 

 body-wall is lined with ciliated coelomic epithelium. 



The skeleton of Holothuria forskali consists chiefly of calcareous 

 spicules, knotted or branching rods, and perforated discs (Fig. II. 

 6, A, B\ secreted by connective tissue cells and forming a thin 

 layer in the outer region of the cutis, especially on the papillae and 

 ambulacral appendages (podia and tentacles). Spicules may also be 

 found in the connective tissue throughout the body, and they are 

 very numerous in the wall of the stone -canal and madreporite. A 

 large perforated plate is situated at the extremity of each podium. 

 A ring of ten calcareous pieces surrounds the oesophagus; the five 

 radial pieces are notched and larger than the interradial (Fig. I. 4). 

 1 This figure refers to the allied species Holothuria tubulosa, Gmeliu. 



