SOME PROBLEMS OF ANNELID MORPHOLOGY. 59 



as a diagram of the common earthworm) is divisible into 

 two widely different regions, though the grounds for 

 making the division do not very clearly appear until the 

 embryological development is taken into account. The 

 first, known as the head or prostoniiiim, lies anterior to 

 the mouth. It is unsegmented, contains no organs of 

 reproduction, excretion, or circulation (minute blood- 

 vessels excepted), and is devoted to the higher functions 

 of sensation and coordination. It contains the brain 

 (cerebral ganglia), and is often, though not always, pro- 

 vided with eyes, antennae, or other highly organized 

 sensory apparatus. The second portion, known as the 

 body or trunk, lies posterior to the mouth. It is much 

 larger than the head, and forms an elongated cylinder 

 divided into a large number of segments {inetameres 

 or somites). The trunk is segmented internally as well 

 as externally, nearly all of the internal organs being 

 divided into segments, or repeated in the successive 

 somites throughout its whole extent — as for example, 

 the ribs or the spinal nerves are repeated in a vertebrate. 

 As regards function, the trunk is in the main given over 

 to nutrition, circulation, excretion, motion, and repro- 

 duction ; its actions are, however, regulated by a series 

 of ganglia, a pair to each somite, that form a "ventral 

 nerve-cord " along the middle line of the body. It is 

 also, as a rule, provided with sense-organs ; these, how- 

 ever, are in most cases less highly organized than those 

 of the head. It has been proved that, in many cases at 

 any rate, the brain exercises a directive action over the 

 other ganglia ; so that from a physiological point of view 

 the body may be regarded as subordinate to the head. 

 We shall find a somewhat similar morphological subordi- 



