692 RADIATA. 



presence of ceVr/j?, or tubular tendril-like suckers, capable of being projected from the surface; of 

 these, a rudiment is to be traced in the existing representatives of the Crinoidece ; they are more fully 

 developed in the Star-fish ; attain their highest development in the Echinus; are less numerous and 

 efficient in the Holothicridce ; and disappear in the Sipunculidce, the softness and flexibility of whose 

 worm-like bodies render them unnecessary for locomotion. 



The minute structure of the skeleton is essentially the same throughout the group ; whether it forms 

 a branching stem with a more or less massive body as in the Crinoidece, a complete globular shell 

 as in the Echinus^ or a regular series of detached plates as in the Star-fish ; or is only represented by 

 a few isolated patches of calcareous deposit as in the Holothuria. It is chiefly composed of carbonate 

 of lime, the proportion of animal matter being very small ; and the material forms a very regular net- 

 work, with open spaces which communicate freely with each other.- The skeleton is thus rendered 

 very light, whilst at the same time it possesses very considerable firmness, each part supporting the rest 

 and deriving support from it. In certain situations, where increased strength is required, it is derived 

 from the interposition of solid ribs oj pillars ; this is the case, for example, in the spines of the Echinus, 

 in which the solid ribs and the intervening net-work are arranged in patterns of great regularity and 

 beauty. [See Dr. Carpenter on the Structure of Shells, (fee, in Report of British Association for 1847.] 

 The classification that seems in most complete harmony with the characters of the group and with 

 its principal varieties of form, is that of Professor E. Forbes [British Echinodermata], which is 

 founded especially upon the organs of locomotion. He divides the entire class into six orders; to 

 which a seventh must now be added, in order to comprehend some very remarkable extinct forms 

 recently discovered. 



I. Cbinoide«. The existing forms included in this order are few ; but it was extremely abundant in former 

 periods of the earth's history, and its remains form no inconsiderable portion of the solid crust of the globe. The 

 one of its representatives whose structure has been most completely investigated, is the Comatula ; an animal 

 which, at first sight, does not depart very widely fi-om the type of the Star-fish, with which it was associated by 

 Cuvier. But it differs from it in several important particulars. The digestive cavity is confined to the 

 central disk, and has two orifices, a mouth and an anus. The arms arising from this disk are solid, being com- 

 posed of the calcareous frame-work already described ; but they are covered with a thick and soft integument, 

 in the substance of which the ovaries are dispersed, forming many thousand distinct spots. The arms are five 

 in number ; but they speedily subdivide, each usually separating into four. To the central stem of each arm, 

 ointed lateral appendages of a similar structure are attached ; and these also are clothed with the fleshy integu- 

 ment, which extends on either side in a sort of fin-like expansion. By the simultaneous movement of the 

 arms, and the stroke of these numerous pirmcB, or fin-like appendages, against the water, the Comatula swims 

 through the ocean very much after the manner of a Medusa. Hence this order may be termed Pinnigrada. 

 Sometimes, however, the Comatula attaches itself to sea-weeds or other floating bodies ; and employs its long 

 arms in bringing food to its mouth. It is not always, however, so completely free in its movements ; for it begins 

 life in the attached condition of a true Crinoid animal, having a long slender stalk which proceeds from the side 

 of the disk opposite to the mouth, and which terminates in an expanded suctorial disk whereby it is fixed on a 

 solid basis. This stem is made up of the same kind of structure as the remainder of the skeleton, and is en- 

 closed by the same irritable integuments, by the contraction of which the head may be made to turn in any 

 direction. When arrived at their full growth, the disk and arms quit the stem, and pass the remaining term of 

 their existence in a state of freedom. A trace of the original attachment, however, may still be detected on the 

 disk. The piivnm are not developed on the arms, until near the close of the period of attachment ; and their 

 membranous expansions are probably peculiar to the free-moving species of this order. The Comatula in its 

 attached state has been described as the Pentacrinus Europccus. It is very minute, and has only been discovered 

 hitherto in the Cove of Cork. 



A much larger Pentacrinus (P. Caput Medusce) has been found, however, in the West Indian seas ; which 

 probably passes its whole life in the attached condition, and is thus a truer representative of the vast assemblage 

 of extinct Crinoids. The disk and arms are formed like those of the Comatula ; the latter are very numerous, 

 and are thickly set with jointed pinme. The stem is more than a foot long, and is composed of a large number 

 of pieces similar to those of the arms. From this stem there arise, at regular intervals, several verticils of se- 

 condary arms, which do not subdivide and are destitute of lateral appendages. The ovaries are not so dispersed 

 as in the Comatula ; but they are still external to the central disk, being seated on the arms near their base. 

 In some of the fossil species of Pentacrinvs, which are especially abundant in the Lias formation, the subdivision 

 and ramification of the arms is carried to a much greater extent than in either of the existing forms. The num- 

 ber of pieces in the skeleton thus becomes very large. In the P. Briareus it has been calculated that at least 

 100,000 exist, exclusively of the joints of the lateral appendages, which are probably more than 50,000 additional. 

 The base of the stem of the recent P. Caput Medusce has not yet been obtained, so that its mode of attachment to 

 solid bodies has not yet been clearly made out ; but from the circumstances under which fossil remains are some- 

 times met with, there is reason to believe that the animals of this genus were not permanently adherent to solid 

 masses, but had the power of occasionally detaching themselves. 



