ANIMAL KINGDOM. 123 



pp. 175, 202, 227) existed only in Paleozoic time. They have jointed 

 bodies with a crust-like exterior, and were aquatic, like all Crustaceans ; 

 and externally they resemble some Tetradecapods (Fig. 164). But 

 they are most closely related to the modern species of Limulus (one of 

 which is the large " Horse-shoe " of the Atlantic coast) ; yet, unlike 

 the Limuli, their legs were very slender or wanting. The name Trilo- 

 bite refers to the division of the body by two longitudinal depressions 

 or sutures, into three parts (Fig. 267); but in some species, this char- 

 acter is indistinct. 



The Limuli differ from other Crustaceans in interior structure and 

 in embryological development ; and they have hence been recently 

 made into a separate group, which includes also, besides Trilobites, the 

 Eurypterids (Fig. 474, page 239). 



In the Trilobite, the shell of the head-portion (a b, Fig. 167) is usually called the 

 buckler ; the tail- (or properly abdominal) shield, when there is one (Fig. 360), the 

 pygidium. The buckler (a b) is divided by a longitudinal depression into the cheeks 

 or lateral areas, and the glabella, or middle area (Fig. 167). The cheeks are usually 

 divided by a suture extending from the front margin by the inner side of the eye to 

 either the posterior or the lateral margin of the shell. In Fig. 167 {Calymene Blu- 

 meribachii), this suture terminates near the posterior outer angle. The glabella may 

 have a plane surface, or be more or less deeply transversely farrowed (Fig. 167), 

 and usually with only three pairs of furrows. 



"Worms or Annelids. — The Annelids include, 1, the Chcetopods, having setae 

 for locomotion ; 2, the Sipunculoids, having the body smooth and cylindrical ; 3, the 

 Bdelloids, or Leeches ; besides the two groups of free-swimming oceanic species, 

 called Chcetognaths (Sagittae), and Gymnocopa (Tomopteris). 



The Chaetopods embrace the groups — 



(1.) Dorsibranchs, or free sea worms, having in general short branchial appendages 

 along the back. Many swim free in the open sea, and others live in the sands of 

 sea-shores or the muddy bottom. The Arenicola family includes species that bur- 

 row in the sands of sea-shores. Fig. 160 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 four feet. 



(2.) The Tubicola, or Serpula tribe, which live in a calcareous or membranous 

 tube, and have a delicate branchial flower, often of great beauty, near the head. 

 They are confined to salt water. The tubes often penetrate corals, and the branchial 

 flower comes out as a rival of the coral polyps around it. 



(3.) The Terricola (Oligochaeta), or Earth-worm tribe, destitute of branchial 

 appendages ; as the common Earth-worm. 



The Helminths, or Intestinal worms, are not here included among Articulates. 



III. MOLLUSKS. 



The three grand divisions of Mollusks are — 



I. Ordinary Mollusks, having usually regular gills or branchice y 

 in addition to an outer enveloping fold of the skin called a, pallium, 

 from the Latin for cloak ; as the oyster, snail, and cuttle-fish. 



II. Ascidian Mollusks. Unlike Ordinary Mollusks in being 



