563 INVMBTEBRATE MORPHOLOQT. 



phiura squamata, e\en serving as brood-pouclies in which the 

 young develop. 



The ectoderm is indistinguishable over the greater portion 

 of the body in the adults, becoming, as in the Crinoids, con- 

 founded with the mesoderm. Calcareous plates are largely 

 developed in this tissue (except in Ophiomyxa and its allies), 

 giving to the disk and arms a brittleness which has suggested 

 the popular name for the group. The extent to which the 

 apical system of plates is distinguishable in the adults varies 

 considerably even in members of the same group, and while 

 in some forms (Fig. 257) all the plates represented in the 

 Starfish Zoroaster can be distinguished, in others only the 

 radials or the basals or both are visible. At the tip of each 

 arm is a plate comparable to the terminal of the Aste- 

 roidea, and in addition there are frequently present series of 

 iuterradials or interbrachials, the most aboral plates of 

 which separate the radials from each other and extend round 

 to the oral surface, abutting on five large plates known as the 

 buccal shields and corresponding to the orals of other forms. 

 On the aboral surface of the disk above the origin of each arm 

 there is a pair of plates termed the radial shields, which 

 must not, however, be confused with the radial plates extend- 

 ing along the aboral surfaces of the arms. 



These latter form a complete series extending from the 

 disk to the terminal plates, and form the aboral wall of the 

 arms, their lateral walls being formed by another series of 

 plates, the adambulacrals (Fig. 258, Ad), while still another 

 series, the superambulacrals, form their oral walls. Between 

 each adambulacral plate and its successor is a pore (usually 

 bounded by a number of small plates) through which the 

 tube-feet are protruded, the radial water-vascular canals being 

 situated in the interior of the arm. The cavity of the arms 

 is occupied almost entirely by a linear series of calcareous 

 masses termed the vertebral or ambulacral ossicles (Figs. 258 

 and 260, A), each of which consists of two halves, usually 

 firmly united by suture. The ossicles are united by well-de- 

 veloped articular surfaces, and have attached to them muscles, 

 whereby a considerable amount of motion is possible for the 

 arms as a whole, the motion being almost entirely in a hori- 



