476 ZOOLOGY SECT. 



cells. The stomodseum and the proctodseum are lined internally 

 by a cuticular layer, which is continuous with the cuticle of the 

 general surface. The anus is usually terminal in position, some- 

 times directed towards the dorsal aspect. There is, in most 

 instances, a longitudinal mesentery running to the alimentary 

 canal from the dorsal body-wall ; sometimes a ventral mesentery 

 is also present bearing a corresponding relation to the ventral 

 surface. 



Some Cha?topoda are entirely devoid of blood-vessels. In one 

 family in which this occurs (the Glyceridce among the Phaneroce- 

 phala), the perivisceral fluid, which assumes some of the functions 

 of the blood, contains numerous red corpuscles, the red colour of 

 which is due to the presence of haemoglobin (see p. 36). In the 

 majority of the Chastopoda there is a highly developed vascular 

 system. Sometimes the blood is colourless : very commonly it 

 is bright red in colour, owing to the presence of haemoglobin, 

 which is not confined to the corpuscles, but is dissolved in the 

 plasma. In Serpula and its allies the blood is bright green, owing 

 to the presence of a green colouring matter, which has an affinity 

 for oxygen similar to that possessed by hemoglobin. 



The chief blood-vessels are usually dorsal and ventral longi- 

 tudinal trunks. These are connected together by metamerically 

 arranged transverse branches. In some of the Cryptocephala the 

 dorsal vessel is not present in the greater part of the length of the 

 body, its place being taken by a circumintestinal sinus or a 

 circumintestinal plexus of vessels lying in the wall of the ali- 

 mentary canal. This gives off in front a short thick- walled dorsal 

 vessel or " heart." The movement of the blood is effected in most 

 instances by peristaltic contractions of the dorsal vessel or of a 

 cirumintestinal sinus or plexus or of a short and wide dorsal 

 cardiac sac given off by the latter anteriorly, which have the effect of 

 driving the blood from behind forwards. In some instances, as in the 

 Earthworms and some Cryptocephala, specially dilated lateral 

 vessels are contractile, and by their pulsations bring about the 

 circulation of the blood through the system of vessels. Plexuses 

 of fine capillary vessels in the integument of various parts 

 frequently aid in respiration, and are particularly well developed 

 in certain forms in which definite organs of respiration are absent. 



The nervous system consists of a cerebral ganglion or brain 

 arid a double ventral chain of ganglia. The cerebral ganglion is 

 distinctly bilobed, and may be looked upon as composed of two 

 intimately united ganglia. It is almost invariably situated in the 

 prostomium, though placed a little further back in the Earth- 

 worms ; it gives off branches to the eyes and tentacles. From it 

 there run backwards and downwards the paired ossophageal con- 

 nectives, which embrace the anterior part of the alimentary canal 

 between them, and below join the anterior end of the ventral chain 



