CRINOIDS. 143 



formed by the upper stem-joints are the numerous cirri with which the animal grasps 

 bodies to which it may desire to become temporarily attached. There are five series 

 of radials, each containing three ossicles. The first or lowest three are closely 

 adherent to each other and to the centro-dorsal tubercle, which conceals them on 

 their outer side. That the central ossicle with the cirri is not the true basal plate 

 is proved by the presence on its upper surface of a basal enclosed between the apices 

 of the first three radials. This basal plate, called the rosette, is formed by the 

 coalescence of the five basals of the larva. 



The alimentary canal makes about a turn and a half round the axis of the body, 

 and ends in the projecting inter-radial rectal cone. Included within the coil of the 

 alimentary canal is a sort of cone of connective tissue, which has been called the 

 columella. The five oral valves contain no calcareous plates in Antedon. The posi- 

 tion of the genital glands in this and other crinoids, namely, in the pinunles, seems 

 very exceptional, yet they are lodged in tissue, which is a continuation of the cellular 

 tissue of the arms, comparable to that in which the ovaries are lodged in star-fishes. 

 The species of Comatulte are numerous. The " Blake " expeditions have latterly 

 added forty to the twenty previously known from the Caribbean Sea, and about 

 seventy were dredged by the " Challenger " between Cape York and the Philippines, 

 and thence southward to the Admiralty Islands. 



The chief distinctions between the two principal genera are as follows : — In Ante- 

 don the mouth is central, or sub-central, and the ambulacra are equal, the arms are 

 equal in length, grooved, and furnished with tentacles. Red spots, the nature of 

 which is unknown, and which are absent in Actmometra, are always present at the 

 sides of the ambulacra. The cirri are numerous, and more or less cover the centro- 

 dorsal tubercle, and the outer faces of the radial plates are relatively high, and inclined 

 to the vertical axis of the calyx. In Actinometra the mouth is not in the centre of 

 the disc; the ambulacra are variable in number and unequal in size, two of them 

 always forming a horse-shoe round the anal area ; the pinnules of the mouth have 

 combs at their tips; some of the hinder arms may be shorter than the others, 

 ungrooved, and without tentacles ; brown spots, thought to be sense-organs, may be 

 present on the dorsal side of the pinnule segments ; the cirri are few ; and the outer 

 faces of the radials are wide and parallel, or nearly so, to the axis of the calyx. In 

 Antedon the ambulacra of the pinnules may be protected by side-plates and covering- 

 plates, but these are absent in Actinometra. 



In the Caribbean Sea Actinometra is represented by more species and more indi- 

 viduals than Antedon, and two-thirds of the species of the latter and three-fourths of 

 the former have ten simple arms. In the remaining species the rays rarely divide 

 more than twice ; only two species divide four times. Antedon and Actinometra are 

 about equally represented in the eastern seas, but while half the species of the foi-mer 

 genus are ten-armed, only three Actinometrce are thus simple. Nearly all the ten- 

 armed Actinometrce of the eastern seas have the second and third radials united by a 

 double joint, without muscles, called a ' syzygy,' and each of the first two brachials 

 is a double or syzygial joint. But all the ten-armed Actinometrce of the West Indies 

 have the second and third radials joined by ligament, whilef the first syzygy is on the 

 third brachial. 



The two Comatulse, which, from their abundance, seem to specially characterize 

 the Caribbean Islands, are Antedon spinifera and Actinometra pidchella. The first 

 has usually thirty arms, but occasionally forks four times. The disc bears a tolerably 



