88 
ZOOLOGY. 
ated among the spines, tlie student will be ready to study the external 
and internal anatomy. 
First, as to the calcareous framework of the star fish. In order 
to study this, a transverse section should be made through an arm, 
and a vertical one through the body and along the middle of a single 
arm, and finally the animal should be divided into two halves, an 
upper and lower. It will then be seen that the calcareous frame- 
work or so-called skeleton consists of a great number of limestone 
plates or pieces attached by a tough membrane and covered by the 
skin. Between the plates are small apertures by which the water 
enters the body-cavity. These plates are arranged so as to give the 
greatest strength and lightness to the body. There is also to be seen 
an oral (under) side on which the mouth is situated, and an aboral 
(upper) side. Each arm or ray is deeply channelled by the "ambu- 
hicral furrow" containing four rows of suckers or "ambulacral feet,' 
which are tentacle-like protrusions of the skin growing out through 
Fig. 37.— Longitudinal section through the body and one arm of Asterias vulgaris. 
m, mouth; s, stomach; lobe of stomach extending into the arm; a, anus; 
nr, nervous ring; n, radial nerve; vi\ water-vascular ring, sending a radial 
vessel {v) into the arm; inp, madreporic plate; t, stone canal; h, haemal 
canal; ov, oviduct; o, ovary; am, ampullae, the ambulacral feet projecting 
below; 6, coeca or liver. 
orifices in the ambulacral plates, and are a continuation of the 
water-sacs or ''ampullae" within. The ''madreporic plate" is a flat- 
tened hemispherical body situated on the disk between two of the 
arms. It is perforated by canals. 
We are now ready to examine the internal organs and to study 
their relations to one another and to the body-walls. The nervous 
system may be seen without dissection. By closely examining the 
mouth a pentagonal ring is seen surrounding it, each angle slightly 
enlarging and sending off a nervous cord to the eye at the end of the 
ray. It may be discovered by pressing apart the ambulacral feet 
along the median line of each arm. Fine nerves are sent off to each 
sucker, passing through the opening between the calcareous plates 
and extending to each ampulla, thus controlling the movements of 
the suckers. The visible nerves belong to a nervous subcutaneous sheet. 
The mouth (Fig. 37, m) is capacious, opening by a short oesophagus 
into a capacious stomach (Fig. 37, s) with thin distensible wal's, and 
sending a long lobe or sac (Fig. 37, I) into the base of each arm- 
