ANARTHROPODA. I31 
above the gullet (fig. 56). 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 ina 
distinct perivisceral cavity, which contains a fluid with solid 
particles 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-hemal” system 
(Gr. Aseudos, falsity ; and Zazma, blood). It is believed, on the 
other hand, to be truly homologous with the water-vascular sys- 
tem of the Axnuloida. Respiration is effected by the general 
surface of the body, or by distinct gills or branchiz. In most 
cases, also, there exists a series of peculiar involutions of the 
integument, which are known as the ‘‘segmental organs” 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 Axnelida may be divided as follows :— 
SECTION A, ABRANCHIATA.—Without gills or branchiz. 
1. Hirudinea.—(Leeches.) 
2. Oligocheta.—(Earth-worms.) 
SECTION B, BRANCHIATA.—Witbh branchie. 
3. Tubicola.—(Tube-worms.) 
4. Errantia.—(Sand-worms.) 
ORDER I. Hirupinea (Déscophora).—This order comprises 
only the leeches, some of which are marine, whilst others in- 
habit fresh water. The leeches (fig. 59) are all characterised 
by the fact that the body is destitute of lateral bristles or foot- 
tubercles, but is provided with a sucking-disc at one or both 
extremities. In the typical forms, as in the common medicinal 
leech, there are sucking-discs at both ends of the body, and in 
those in which only the hinder sucker is present, the head can 
