144 LOWER INVERTEBRATES. 



complete plating, and there is a double row of plates along each edge of the pinnule 

 ambulacra, which are thus covered over. The spread of the arms is about eight 

 inches. Actinometra pulchella varies greatly. The arms may be ten, twelve, or 

 even twenty. The mouth is far out of the centre, and the disc is bare, or more or 

 less covered with calcareous plates. It is about ten inches across the arms. 



Atelecrinus, of which two species, cubensis and hcdanoides, are known, has the 

 first radials visible, and separated from the centro-dorsal tubercle by a complete circlet 

 of basals. No other Comatula, with one doubtful exception, retains its embryonic 

 basals on the exterior of the calyx after the latter part of its existence as a Pentacri- 

 noid, and no other Comatula, recent or extinct, has a complete basal circlet of five 

 pieces. 



In A. halanoides the acorn-shaped centro-dorsal is marked all over by the horse- 

 shoe-shaped sockets of the cirri. The first ten or twelve of the arm-joints are with- 

 out pinnules. In the characters of the calyx Atelecrinus is a permanent larva. 



The Comatulae dredged by the " Blake," were nearly all taken in depths less than 

 200 fathoms ; and as the " Challenger " only found Comatulse at greater depths than 

 this on twenty occasions, P. H. Carpenter concludes that the group is essentially 

 a shallow water one. 



We now come to the stalked crinoids, which, though they so long eluded search, 

 are not really abyssal forms, since they have on only fourteen occasions been dredged 

 from depths exceeding 650 fathoms. 



Pentacrinus has a long stem, bearing whorls of unbranched cirri at intervals, and 

 lives attached to rocks in moderate depths. The joints of the stem are pentagonal. 

 There is no distinct basal piece to the calyx, and the arms bifurcate twice, thus giving 

 twenty principal free arms. The first forking occurs at the third rg-dial. The ambu- 

 lacral grooves of the arms have a series of ossicles along their floor and lamelliB along 

 their margin. 



In this genus the third i-adials, or radial auxiliaries, have their two sides bevelled off 

 like the eaves of a gable, to allow two joints, the first joints of the ten arms, to be 

 seated upon them. The first joints of the arms are split in two by a peculiar 

 joint called a ' syzygy.' The ordinary arm joints are provided with muscles for the 

 various motions, but the syzygies are not so provided, and consequently, when one of 

 the arms is entangled, or caught, it breaks ofE at one of the syzygies, a beautiful pro- 

 vision for the safety of an animal with so comijlicated a crown of appendages. 



The stem of Pentacrinus consists of many flattened calcareous joints, the upper 

 and lower surfaces of each of which show five radiating leaf-like spaces, each sur- 

 rounded by a border of tiny ridges and grooves. These ridges and grooves fit into 

 each other, so that, in spite of the number of joints, the motion of the stem is very 

 limited. The leaflet-like spaces thus come over each other, and five oval bands of 

 strong fibres pass right through the loosely-arranged calcareous network of these 

 spaces from one end of the stem to the other. There are no muscles between the 

 joints, so that the animal does not appear to be able to move its stalk at will. Each 

 joint has a hole in its centre, and this chain of holes is continuous with others which 

 run through to the plates of the cup, and continue through the axis of each arm, and 

 along every pinnule. In P. asteria about every seventeenth joint of the stem bears a 

 whorl of fine, long tendrils, or cirri, which start from shallo\v grooves in the pro- 

 jecting angles of the pentagonal stem. These cirri usually have thirty-six or thirty- 

 seven short joints, the last of which is sharp and claw-like. Though they have no 



