ZOOLOGY. 



this order, is covered with a tough leathery skin 



beset with prickles. 

 The animal has 

 the form of a star 

 with five or more 

 rays radiating from 

 a central disc. In 

 the middle of the 

 under surface of 

 the disc is situated 

 the mouth, open- 

 ing into an intes- 

 tine, which sends 

 prolongations into 

 each ray. In some 

 forms there is no 

 Asterias tessellata (Asteriada). anus present, so 

 that the indigest- 

 ible portions of the food have to be ejected 

 through the mouth, as in the common sea- 

 anemone. If the prickly skin be removed, it is 

 seen that it is supported by a series of plates, 

 beautifully jointed together. On the under sur- 

 face of each ray, the plates exhibit a series of 

 perforations, through which, in the living state, 

 the ambulacral feet can be protruded. They are 

 found in almost all tropical and European seas, 

 and some species are found as far north as 

 Greenland. 



ORDER 4. Echinoidea. The members of this 

 order are commonly known as Sea-urchins or Sea- 

 eggs. The body is somewhat globose in form, 

 and composed of a series of plates jointed to- 

 gether. In the Echinus, we 

 observe two orifices situated 

 at the two poles of this 

 globe ; the larger of these 

 orifices, directed downwards, 

 is the mouth ; at the smaller 

 one, placed superiorly, the 

 intestine terminates. The 

 mouth is furnished with a 

 curious apparatus of teeth, 

 known as 'Aristotle's Lan- 

 tern,' worked by a powerful 

 set of muscles attached to 

 the edge of the shell near the 

 mouth. By the action of the 

 teeth the food is ground down 

 before it passes into the in- 



Shell of Echinus : 



a, in ter-amhulacral plates 

 6, ambulacral plates. 



testine, which takes a couple of turns round the 

 shell before its termination. Round the second 

 orifice of the shell are disposed the ovaria, which 

 are very largely distended with eggs at some 

 seasons, and are eaten under the name of roe of 

 the sea-egg. On looking at the Echinoidea in the 

 living state, we see that most of them are covered 

 with spines of considerable size. Moreover, these 

 spines are movable at the base, which is 

 hollowed out into a little cup, which fits on a 

 rounded projection from the shell, thus forming a 

 complete ball-and-socket joint These spines are 

 connected to the shell, and are moved by the 

 skin which covers the latter. On looking at the 

 shell of an Echinus, it is seen to be composed of 

 twenty rows of plates, generally hexagonal in 

 form, accurately fitted to each other, running 

 from pole to pole. They are so arranged that 

 there are ten alternating zones, each zone being 

 composed of two rows of similar plates. There 

 are five double rows of large plates, which are 



imperforate, and studded with tubercles. These 

 are the inter-ambulacral areas. The other five 

 double rows (ambulacral areas) alternate regu- 

 larly with these, and are composed of small 

 plates, which are perforated by numerous aper- 

 tures for the protrusion of the ambulacral feet. 



These feet are distended by water admitted 

 from without through a specially modified plate, 

 called the madreporiform tubercle, which admits 

 it into the sand-canal, a straight tube filled with 

 peculiarly shaped spicules, which finally opens into 

 a ring-like vessel surrounding the oesophagus, and 

 sending off tubes in a radiate manner, from which 

 the ambulacral feet spring. When water is ad- 

 mitted into these feet, they become distended, and 

 are protruded through the ambulacral apertures. 

 These feet are provided with a small sucker, by 

 which they can adhere to the rocks, thus enabling 

 the animal to move along. Attached to and 

 communicating with the ring-like vessel sur- 

 rounding the oesophagus, are two little sacs or 

 bags, called Polian vesicles, which act as reser- 

 voirs for the water, so that the ambulacral feet 

 can be protruded at will. The spines are most 

 numerous in the inter-ambulacral areas. In order 

 to provide for the growth of the shell, there is 

 interposed between adjoining plates a thin mem- 

 brane, by the calcification of which the plates 

 increase in size at their edges. These animals are 

 found on sandy shores, and on the bottom of the 

 sea, creeping along with their feet. Their food is 

 of a mixed character, consisting of Crustacea and 

 sea-weed. In some forms, as Spatangus (Heart 

 Urchin), the shell is ovoid or heart-shaped, and 

 the mouth is eccentric. In these cases, the dental 

 apparatus is generally imperfectly developed. 



ORDER 5. The Holothuridea, the last and most 

 highly organised Echinoderms, are destitute of 

 spines or prickles, and have the body shaped like 

 a cucumber, and inclosed in a highly elastic skin. 

 They are commonly known by the name of sea- 

 cucumbers. Some of the species are edible, and, 

 when dried, are the trepang of commerce. By 

 the Malays, they are diligently sought after for the 

 supply of the Chinese market 



IV. SUB-KINGDOM ANNULOSA. 



The Annulosa, or Ringed Animals, have their 

 segments arranged, one behind the other, from 

 before backwards. They exhibit bilateral sym- 

 metry, each segment being also symmetrical. The 

 alimentary canal, which is present in all except a 

 few internal parasites, is distinctly shut off from 

 the body cavity. In most, a blood-vascular 

 system is present, and is always placed dorsally. 

 In the higher orders, the nervous system consists 

 of a double symmetrical rord, placed along the 

 ventral surface of the animal, and having upon 

 it small swellings composed of nervous matter, 

 and termed ganglia, two ganglia corresponding to 

 each segment These ganglia' give off branches 

 to their respective segments. Anteriorly, the 

 gullet is surrounded by these nervous cords, 

 so that it passes through a ring of nervous 

 matter. 



The Annulosa are divided into two principal 

 divisions : I. Vermes, and II. Arthropoda. 



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