128 INVERTEBRATE ANIMALS. 



place of the proper vascular system, and which are known as the 

 " pseudohseraal " vessels. 



The two principal gro)ips included in this division of Annulose 

 animals are the (Jcpliyrea (Spoon- worms) and the Annelida (Ringed 

 Worms). 



Class I. Gephyrea. 



This is a small group, including a number of worm-like animals, 

 commonly called Spoon-worms, which in many respects resemble 

 the Annelides, and have at the same time certain relationships with 

 the Holothurians among the Echinoderms. They differ from the 

 Annelides in not having the body definitely segmented, though the 

 skin is aiinulated (fig. 80), and also in the fact that the ventral 

 neiTe-chain is not gangliated. Theie are no lateral appendages, 

 except occasiouallv biistles. They differ from the Holothurians in 

 the want of an anibulacral system and the non-radiate character of 

 the nervous system. The Spoon-worms are all marine, and the most 

 widely distributed genus is Sipunculus (fig. 80), which ranges from 

 between tide-marks down to a depth of 2500 fathoms. 



Class II. Annelida. 



Yhe Annelida or J /i/i«/ato, commonly known as Ringed Worms, 

 are distinguished by the possession of definite segmentation, the 

 body being composed of a number of rings which are all similar to 

 each other except at the two ends of the body. All the Annelida 

 are moie or less worm-like in shape, and in all except the Leeches, 

 the segments are (some or all) provided with lateral appendages 

 which mostly subserve locomotion, but which are never jointed to 

 the body. In the t3-pical Annelida each segment (fig. 81) consists of 

 two arches, termed, from their position, respectively the " dorsal 

 arc" {<l), and the " ventral arc" {v). Each segment carries a lateral 

 process on each side, which are known as the " foot-tubercles " 

 {parapodia). Each foot-tubercle in turn may consist of an upper 

 piece or "dorsal oar" («), and a lower piece or "ventral oar" (6), 

 both carrying a tuft of bristles and a soft jointed filament (the 

 " cirrus ''). 



The nervous system consists essentially of a double gangliated 

 chain placed along the ventral surface of the body, and traversed in 

 front by the gullet, so that the first pair of ganglia lie above the 

 gullet. The digestive system consists of a mouth, generally with a 

 protrusible jiroboscis, and sometimes horny jaws, a gullet, stomach, 

 intestine, and a distinct anus. As a rule, the alimentary canal runs 

 straight from one end of the body to the other without describing 



