BRIEF REVIEW OF THE SYSTEM OF LIFE. 



428 





Decapods 

 (Grab). 



Tetradeca- 



PODS. 



Cyclops. 







Cbphalothoeax. 



Abdomen. 





I 



II in 



IV V VI vn vm ix x xi xn xiii xiv 



I n m IV V VI 



1. 



Pedunc. 



eyes. 





 



2 pairs 



of 

 antennje. 



6 pairs of mouth 5 pairs of feet, 

 organs. 



6 pairs of 



abdominal 



appendages. 



2. 



2 pairs 



of 

 antennae. 



2 pairs 



of 



antennae. 



4 pairs of 7 pairs of feet, 

 mouth 

 organs. 



6 pairs of 

 abdominal 

 appendages. 



3. 



3 pairs one 4 pairs of 



of pair natatory 

 mouth feet. feet, 

 organs. 



usually no 



appendages 



except to last 



segment. 



2. Worms ( Vermes) . 



Worm-like in form, consisting of many segments not always distinct, without jointed 

 legs, though often furnished with tubercles, lamellte, or bristles. Examples: the Earth- 

 worm, marine Annelids, Leeches. Among the Annelids or higher Worms, the Arenicola, 

 or Sand-worm family, includes species that burrow in the sands of seashores ; Fig. 376 

 represents the A. marina, or Lob-worm, which is common on European and American 

 shores, and grows to the size of the finger. One species of Eunice has a length of 4 feet. 

 They are supposed to be related to the Scolithus of the Cambrian (Potsdam Sandstone). 



Species of Tnbicolce, of the Serpula tribe, live in a calcareous or membranous tube, 

 and have a delicate branchial flower, often of great beauty, near the heads. The tubes 

 often penetrate corals, and the branchial flower comes out as a rival of the coral polyps 

 around it. 



The Botifers are generally made a subdivision of the Worms. They are minute species, 

 having 3 to 6 body segments ; 1 or 2 simple eyes ; a pair of jaws ; disks, situated anteriorly, 

 which are edged with movable cilia in place of limbs. Many have, in appearance, the 

 cephalothorax and jointed abdomen of an Entomostracan, and in this and other ways 

 show a relation to Crustaceans. They are supposed by Lankester to have comprised the 

 precursor species of Annelids, Crustaceans, Limuloids, and other Arthropods ; and others 

 compare the forms of some with the embryos of Mollusks, Molluscoids, and Holothurians, 

 — relations that would make the group the Emhryonoid division of the higher Inverte- 

 brates. For figures of Rotifers and references see article Rotifers in the Encycl. Brit. 



The Helminths, or Intestinal Worms, need no especial remarks in this place, as they 

 have no geological importance. 



3. Mollusks. 



Mollusks consist essentially of a soft, fleshy bag containing the stomach and viscera, 

 without joints or jointed appendages. They were named Mollusks from the Latin mollis, 

 soft. They have on either side a thin fold of the skin of the back, called the mantle or 

 pallium (from the Latin for cloak), which serves to inclose a cavity between it and the 

 body, where are the gills (branchiae) or aerating organs. The mantle varies from very 

 large to nearly obsolete ; and in some (the Pulmonates or land-snails) it is a covering for 

 an internal lung-like organ of respiration. The ventral surface anteriorly has sometimes 

 a firm, fleshy projection which serves as a foot for locomotion, as in the Clam, or for their 

 attachment by homy fibers, as in the Mussel. Again, it is sometimes spread out flat, 

 making a large, flat foot or ventral surface for locomotion, as in the Gastropods ; or it has 

 the anterior part divided into a pair of wing-like paddles, as in the Pteropods ; or into 4 



