1 92 DESCRIPTIONS OF PREPARATIONS. 



the corresponding ambulacral ossicle and with its fellow in the adjoining 

 ray forms two teeth. The ambulacral is in this case a support for the teeth. 

 The first type of mouth is termed ambulacral, the second adambulacral. 

 The former appears to be characteristic of those Starfish in which the ambu- 

 lacral pores are arranged in zig-zag-, the latter of those in which they 

 maintain a linear arrangement. A small plate, somewhat indistinct in this 

 specimen, overlies interradially each set of first adambulacrals. It is known 

 as the odontophore, and appears to be homologous with the oral plate of 

 many other Echinoderms. See the general account of the Phylum. A 

 single row of minute adambulacral ossicles articulating immediately with 

 the outer ends of adjoining ambulacrals can be made out in nearly the 

 whole length of the arm ; as well as the transverse rows of five inter- 

 mediate ossicles, which unite them with a longitudinal row of inferior 

 marginal ossicles bordering the ventral aspect of each arm. The median 

 intermediate ossicle of the five is enlarged and spine-bearing, and is con- 

 nected to the intermediate ossicle in front and behind. In some Starfish, 

 e. g. Astrogonium, there is a series of well-developed dorsal superior marginal 

 ossicles. 



The perisoma or integument consists of two layers, an outer and an inner, 

 between which exists a system of irregular channels which are ultimately continuous 

 with the system of perihaemal spaces. The calcareous ossicles, spines and pedi- 

 cellariae, belong to the outer layer with the single exception of the ambulacral 

 ossicles which belong to the inner one. The outer layer passes across the ambu- 

 lacral groove from side to side, inclosing a space between itself and the ossicles. In 

 this space are lodged (proceeding from the dorsal to the ventral wall) the water- 

 vascular radial canal, the transverse ambulacral muscles, the right and left- 

 perihaemal space with the bloodvessels, and the nerve-forming ectoderm. 



The outer surface of the perisoma is covered by a ciliated epithelium (ecto- 

 derm) which possesses a distinct cuticle pierced by pores for the cilia. The consti- 

 tuent cells are (i) supporting cells, (2) gland cells, (3) sense-cells. At the base 

 of the epithelium is a network of nerve-fibrils with ganglion cells. In the ambulacral 

 nerve and circum-oral nerve-ring the supporting and sense-cells are of great length, 

 and the nerve-fibre layer much thicker, and the fibres parallel to one another. These 

 structures are continued on to the terminal feeler or tentacle, the first formed tube- 

 foot of the arm, as well as on to the paired feet; but in the latter the nerve-fibres are 

 arranged in longitudinal bundles, and gland-cells occur plentifully on the terminal 

 sucking disc. The eye-speck is placed on the ventral surface of the feeler and is com- 

 posed of several eyes, the number increasing with age. Each eye consists of a 

 conical depression in the epithelium over which the cuticle passes uninterruptedly. 

 The walls of the depression are formed of sense-cells, some of which contain pigment, 

 and its central cavity is filled with a clear liquid. If the radial nerves are divided 

 close to the ring, the animal loses the power of coordinating the movements of the 

 several arms : and if the eye-specks are removed, it ceases to react to light. 



The pedicellariae in the Asteroidea possess only two blades with the exception 



