THE STELLEROIDEA 



241 



FIG. II. 



Asterias rubens, part of abac- 

 tiual skeleton. 



skeleton, of Perrier (1884 and 1894), (38), on the pedicellariae, 

 and of Sladen (1889), (48). 



Structure of a Typical Asteroid. The common English star- 

 fish (Asterias rubens, L.) is a convenient type of the Asteroidea. It 

 has a flattened body composed of a central disc from Avhich radiate 

 five arms (Fig. I.). The upper or abactinal surface of the Body is 

 covered by an integument or perisoma (Fig. II.), composed of a 

 network of calcareous rods, the meshes between which are closed 

 by tough membrane. The anus opens almost 

 in the centre of the abactinal surface. 

 Between the anus and one of the angles 

 between the rays occurs the madreporite, a 

 thick grooved plate, perforated by pores 

 leading to the water-vascular system. 



The ventral surface of each Arm is tra- 

 versed by a broad groove ; since the grooves 

 radiate from the mouth to the ends of the 

 arms, the ventral surface is known as the 

 actinal surface. The mouth is at the centre 

 of this side of the body, and is surrounded by spines (the " mouth 

 papillae "). The grooves are occupied by four rows of suckers or 

 podia, and therefore correspond to the ambulacral areas of the 

 Echinoid. On either side of the grooves are three rows of spines. 

 Dissection is necessary for the recognition of any further points 

 in the structure of the Asterias. By the removal of some of the 

 podia, the ambulacral grooves may be seen to lie outside a series 

 of pairs of narrow plates the ambulacral ossicles (Fig. III.). The 



two series of ossicles meet 

 in the middle line ; laterally 

 they abut against a row 

 of adambulacral ossicles, be- 

 yond which are further rows 

 of interambulacral and mar- 

 ginal ossicles, all of which 

 are comparatively small. The 

 ossicles are protected by spines and pedicellariae similar to, but 

 simpler than, those of Echinoidea. 



The Oral Skeleton (or actinostomial ring) consists of a solid 

 calcareous ring around the mouth. It is composed of thirty plates 

 in a quinqueradiate starfish, there being always six times as many 

 plates as there are rays. Each segment of the oral skeleton con- 

 sists of two pairs of ambulacral, and of one pair of adambulacral 

 ossicles. In Asterias the ambulacral plates are more prominent 

 than the adambulacrals, and project into the oral cavity. The 

 mouth armature is therefore on the ambulacral type (Viguier, 

 52). 



16 



Fio. III. 



Asterias rubens, ambulacral and adambulacral plates. 

 , apertures for podia. 



