VERMES; ARTHROPODA 947 



and a ventral one, placed typically on elevations or parapodia. The 

 head segment carries appendages varying in the different sub- 

 classes. Aquatic worms possess gills for breathing, but these be- 

 come more or less modified or even entirely lost in the mud- and 

 earth-worms. The alimentary system consists of an anterior mouth, 

 an intestinal canal, divisible into fore-gut, mid-gut and hind-gut, and 

 ending in the posterior anus. In some parasitic forms, this system 

 is much degenerated. In some chcTtopods a series of horny oesoph- 

 ageal teeth is developed, and these are often preserved in great per- 

 fection. The conodonts may be of this order. 



Many worms build tubes of agglutinated sand, either free, or 

 in the sand, while others secrete calcareous tubes. These are often 

 well preserved and show the presence of these organisms in Cam- 

 bric times. Trails left by errant worms on mud and the peculiar 

 form of the string of sand, which has passed through the annelid 

 body, all serve as evidence of the existence of the worms in former 

 periods. 



Phylum VIII — Arthropoda. The arthropods, or jointed- 

 legged invertebrates, comprise a number of distinct assemblages of 

 organisms, as indicated by the several classes included. The crus- 

 taceans are in many respects the most characteristic, but even they 

 comprise a number of subclasses of very diverse characters. The 

 Myriopoda and Peripatus are worm-like. The former occur first 

 in the Old Red vSandstone (Devonic) and are common in the Car- 

 bonic. The oldest, and in some respects the most generalized, of 

 the Crustacea are the Trilobites, which are already highly developed 

 and very numerous in the Cambric. They do not extend beyond 

 the Palaeozoic. The organism is covered by a chitinous exoskeleton 

 in which a head or cephalon, a thorax and an abdomen or pygidium 

 are distinguishable. Each division consists of a median axis and 

 lateral lobes and hence shows a trilobate division. The axis of 

 the head constitutes the glabella and the lateral portions are com- 

 monly divided into fixed and free cheeks, the latter generally carry- 

 ing the compound eyes. The thorax is divided into a number of 

 movable rings, but the pygidium is a single though grooved piece. 

 The mouth is ventral and the head is provided with antennae. 

 Jointed thoracic legs were also present. The Entomostraca are 

 modified crustaceans with a shell-like carapace. They are repre- 

 sented in all geological horizons. The Ostracoda, with a bivalve 

 shell, were especially abundant in the Palaeozoic. The barnacles 

 also had representatives in the Palaeozoic but are more typical in 

 later horizons. The animal is degenerate, attached either directly 

 or by a fleshy stalk. In the former case a circle of shell-plates is 



