CHAMBERS'S INFORMATION FOR THE PEOPLE. 



includes the Arenicola piscatorum, or Lob-worm, 

 the Nereis (Sea-centipede), and the Sea-mouse 

 (Aphrodite). The respiratory organs are in the 

 form of tufts of branchiae, which are placed along 

 the back or sides of the creature. The back of 

 the Sea-mouse is covered by a series of membran- 

 ous plates, protecting the bristles of the feet, which 

 exhibit beautiful iridescent metallic colours. 



DIVISION II. ARTHROPODA, 



in which the division of the body into segments is 

 well marked, and jointed appendages articulated to 

 the body are present. Most of the segments and 

 appendages are protected by an outer covering, 

 often composed of chitine, which forms a sort of 

 exo-skeleton. The appendages are hollow, and 

 the muscles are prolonged into their interior. The 

 nervous system exhibits the well-marked Annulose 

 type. Respiration is performed differently in the 

 different groups. The vascular system, when 

 present, is always placed dorsally. 



The Arthropoda (Gr. arthros, a joint, andpous, 

 a foot) are divided into four great classes namely, 

 the Crustacea, the Arachnida, the Myriapoda, and 

 the Insecta. 



CLASS i. Crustacea. In a typical crustacean, 

 the body consists of twenty-one segments, seven of 

 which belong to the head, seven to the thorax, and 

 seven to the abdomen. But sometimes the head 

 segments unite with those of the thorax, and form 

 a cephalo-thorax. Some of the segments may be 

 suppressed. Some or all of the segments are pro- 

 vided with a single pair of articulated appendages. 

 The animals are fitted for life in water, and they 

 therefore breathe by gills, or by the general surface 

 of the body. The head carries two pairs of jointed 

 antennae or feelers. The appendages for loco- 

 motion are borne by the thoracic segments, and 

 usually by those of the abdomen also. The body 

 is protected by a calcareous or horny covering or 

 ' crust' They all undergo a series of metamor- 

 phoses or changes before they reach the mature 

 condition. 



ORDER i. Ichthyophthira and Rhizocephala, or 

 the Parasitica of Lamarck in their embryonic 

 condition are furnished with eyes and antennae, 

 and swim freely about ; but the adult is parasitic 

 on the eyes and gills of fishes, to which it adheres 

 by a suctorial mouth. 



ORDER 2. Cirripe- 

 dia (Lat cirrus, a ten- 

 dril, and pes, a foot) 

 represented by the 

 Acorn-shells and Bar- 

 nacles. In the adult 

 condition, they are 

 permanently fixed, 

 and are usually seen 

 attached to rocks, 

 pieces of floating tim- 

 ber, or the bottoms of 

 ships. Several of the 

 head segments are so 

 modified as to form a 

 multivalve shell, in- 

 closing the animal, 

 from which it can 

 protrude itself. The thorax is provided with six 

 pairs of ciliated limbs, which, being protruded 



142 



Barnacle. 



through the opening in the shell, and creating 

 currents in the water, bring food to the animal 

 The embryo is a free-swimming form, which is 

 provided with one eye, has two pairs of antennae, 

 and six limbs, which it uses for swimming. In 

 the barnacles (Lepadida), the animal is fixed by 

 a long stalk or peduncle ; while the acorn-shells 

 (Balani) are sessile, having no stalk. 



SUB-CLASS Entomostraca. Those crustaceans 

 which possess more or fewer than fourteen thor- 

 acico-abdominal segments are included under the 

 sub-class Entomostraca, which includes five orders. 



ORDER 3. Ostracoda for example, the genus 

 Cypris, which abounds in every pool of fresh water. 

 It is a small animal, protected by a shell com- 

 posed of two valves, which it can open and shut at 

 will. It has two or three pairs of feet, and the 

 branchiae or gills are attached to the posterior 

 jaws, which surround the mouth. In a fossil state, 

 their shells are found abundantly in the Wealden 

 rocks of England, in the Carboniferous Lime- 

 stones, &c. 



ORDER 4. Copepoda for example, Cyclops, one 

 of the ' Water-fleas/ which is com- 

 mon in all our ponds and ditches. It 

 is of small size, and the head and 

 thorax are protected above by a 

 structure called a carapace. It has a 

 single large eye, placed anteriorly, 

 and two pairs of antennae. Five pairs 

 of feet are present, and are used for 

 swimming. 



ORDER 5. Cladocera represented 

 by Daphnia pulex, another of the 

 Water-fleas. The head is distinct, 

 and has a single eye, and the remainder of the 

 body is inclosed in a shell. 



ORDER 6. Phyllopoda (phyllon, a leaf, &rApous t 

 a foot) includes the genus Apus and the Fairy 

 Shrimp, in which the body is protected by a 

 carapace. They have always more than sixteen 

 feet, which carry the gills ; and the eyes are well 

 developed. 



An order, the Trilobita, now entirely extinct, 

 was probably allied to the Phyllopoda. The body 

 was covered by a chitinous shield, consisting of a 

 cephalic shield, a variable number of body-seg- 

 ments, and a tail with the joints more or less 

 anchylosed, the whole shewing a division into 

 three longitudinal lobes, hence the name. The 

 eyes were sessile and compound, and the number 

 of facets in the lens of one species numbered 6000. 

 They were entirely confined to the Palaeozoic 

 rocks, and over 200 species have been described 

 from the Silurian rocks alone. 



Cyclops vul- 

 garis, mag- 

 nified. 



Asaphus tuberculatus. 



Limulus Polyphemus. 



ORDER 7. Xiphosura (xiphos, a sword, and 

 oura, a tail) represented at the present day by 



