ANARTHROPODA. 121 



traversed in front by the gullet, so that the first ganglion lies 

 above the gullet (Fig. 44). The digestive system consists of 

 a mouth, generally with a protrusible proboscis, 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 any convolutions in 

 its course. In almost all cases the alimentary tube is placed 

 in a distinct cavity, which contains a fluid with solid par- 

 ticles in it, believed to correspond to the blood of the higher 

 Annulosa. In most, if not in all, there is an additional system 

 of vessels which carry a fluid containing solid particles, which 

 are contractile, and which send branches to the respiratory 

 organs, when these exist. This system is believed not to cor- 

 respond to the blood-vascular system of the higher animals, 

 and it has, therefore, been termed the " pseudo-haemal " system 

 (Gr. pseudos, falsity ; and haima, blood). It is believed, on 

 the other hand, to be truly homologous with the water-vascu- 

 lar system of the Annuloida. Respiration is effected by the 

 general surface of the body, or by distinct gills or branchiae. 

 In most cases, also, there exists a series of peculiar involutions 

 of the integument, which are known as the " segmental or- 

 gans " or " respiratory pouches," and which are believed to 

 be partially concerned in the respiratory process. The sexes 

 in the Annelida are sometimes distinct, sometimes united in 

 the same individual. The embryos are almost always ciliated, 

 and many of them pass through a metamorphosis. 

 The Annelida may be divided as follows : 



SECTION A. ABRANCHIATA. Without gills or branchiae. 



1. Hirudinea. (Leeches.) 



2. Oligochceta. (Earth-worms.) 

 SECTION B. BRANCHIATA. With branchiae. 



3. Tubicola. Tube- worms.) 



4. Errantia. (Sand-worms.) 



ORDER I. HIRUDINEA (Lat. hirudo, a horse-leech). This 

 order comprises only the leeches, some of which are marine, 

 while others inhabit fresh water. The leeches (Fig. 47) are 

 all characterized by the fact that the body is destitute of 

 lateral bristles or foot-tubercles, but is provided with a suck- 

 ing-disk at one or both extremities. In the typical forms, as 

 in the common medicinal leech, there are sucking-disks at 

 both ends of the body, and in those in which only the hinder 

 sucker is present, the head can be converted into a suctorial 



