EXERCISE IL. 
THE ROCK STAR-FISH (Asterina vegularis). 
Sub-kingdom—HCHINODERMATA. Class—STELLERIDA. 
1. External form.—Place the animal, oral surface down- 
ward, in a saucer-full of salt water, and turn it so 
that the madreporic tubercle is next to you. Observe 
the anus—slightly to the left of the centre.* Notice 
the dermal plates or ossicula, covered with short blunt 
spines, arranged transversely, in more or less curved. 
rows over the disc, and in oblique rows near the 
margin. Observe, with the dissecting microscope, 
the delicate membranous tubules which project 
through the pores between the ossicula, and move 
about. These belong to the vascular system, and 
are probably respiratory in function. There are no 
pedicellariz. Turn the animal on its back, and 
observe the five radiating ambulacra, each with four 
rows of tube-feet and bordered on either side by a 
single row of spines. 
2. Internal structure.—Remove the animal from the 
water and pin it, oral side downward, to a leaded 
cork. With a knife make a transverse cut through 
the integument, or perisoma, near the tip of one of 
the rays, and carry this incision all round the animal 
above the flat part of the inter-radial spaces. 
* The anus is more readily seen in light-coloured specimens than in 
dark ones, but it is present in all, 
