ECHINODERMATA. 



33 



firmly joined to one another, form by, their 

 union a shell approaching more or less to a 

 spherical figure, (fig. 10, A, B.) The shell is 

 covered outside by a membranous integu- 

 ment, spines, and other appendages; on the 

 inside it is lined by the peritoneum. It is 



Fig. 10. A. 



Echinus esculentus opened, intestine removed. 

 A, under half of shell. B, upper half, a, oeso- 

 phagus cut. b, termination of the intestine. 

 c, c, c, ovaries, d, d, vesicular laminae of the 

 feet. At e, e, the laminae are removed to show 

 the perforations for the feet. 



perforated above for the anal orifice of the 

 intestine (6), and below it presents a much 

 larger opening, which is closed by the mem- 

 branous integument, except in the middle, 

 where the mouth is situated (jig. 15). The 

 pieces composing the shell are mostly five- 

 sided, transversely oblong, and disposed in 

 twenty vertical rows or columns, which extend 

 from the anus to the inferior opening. Ten of 

 the columns are narrower, and consist of smaller 

 pieces, (Jig. 10, e, e,) which are perforated 

 with holes for the feet ; they are thence termed 

 ambulacral. The other ten are broader, and 

 consist of larger pieces (f,f). The ten am- 

 bulacral columns are disposed in five pairs, 

 with which the ten larger columns, also dis- 

 posed in pairs, alternate. The two columns of 

 each pair are joined by a zigzag line. The 



VOL. II. 



upper ends of the columns are connected with 

 ten plates, alternately larger and smaller, placed 

 round the anus ; the larger perforated for the 

 passage of the oviducts, and named ovarial 

 plates, the smaller also perforated by a smaller 

 hole, which is connected with the vascular sys- 

 tem. At its lower edge the shell sends inwards 

 a process in form of an arch over each pair of 

 the ambulacral columns (g, g, g). The number 

 of plates in a row varies with the age of the 

 animal, increasing as it grows older and larger. 

 They are marked on the outside with tubercles 

 or knobs, of various sizes, which support the 

 spines. The spines themselves have a cup-like 

 cavity at their base, which is connected with 

 and moves on the prominent tubercle, the 

 union being effected at the circumference of 

 the articulation by the soft irritable integu- 

 ment, or, according to some, by distinct mus- 

 cular fibres. 



Besides the spines, there exist on the external 

 surface of the Echinus appendages (fig. 11), 

 of the same nature as the claw-like organs of 

 the Asterias, only that in the Echinus the sort 

 of forceps which they bear at their extremity 

 for the most part consists of three blades. 



Jfe.il. 



The shell of the irregularly-shaped Echinida 

 differs considerably in structure from that of 

 Echinus. The division into plates is less ob- 

 vious, and in some cases disappears altogether. 

 The series of holes or ambulacra do not extend 

 uninterruptedly from the anus to the lower 

 orifice. Lastly, in Clypeaster the shell is di- 

 vided interiorly, by vertical calcareous parti- 

 tions, into five compartments which commu- 

 nicate together, the septa being incomplete. 



c. The integuments of the Holothuriae differ 

 considerably in different species. In those 

 species in which there is a marked distinction 

 of the dorsal and ventral surface of the body, 

 the integument differs in character on these 

 two surfaces : in other cases it is pretty nearly 

 uniform over the whole body. It in general 

 consists of a white fibrous layer, which consti- 

 tutes its chief thickness, and a soft coloured 

 layer and epidermis placed more exteriorly. 

 In some species the skin exhibits hard conical 

 warts scattered over the dorsal surface ; in others 

 it contains imbricated calcareous scales. In 

 H. phantapus, in addition to these scales, 

 which are about a line in breadth, the in- 

 tegument, according to our observation, is 

 thickly beset with small calcareous eminences, 

 about -^5 of an inch in diameter, resembling, 

 except in size, the short calcareous processes 

 on the upper surface of the Asterias. 



A calcareous ring, forming in many species 

 the only hard part of the body, surrounds the 



D 



