us 



ANNELIDA. 



Whether the prestomium should be regarded as a segment or not is a disputed 

 question, and one which can only be settled by a study of development. Un- 

 fortunately Embryology speaks with an uncertain voice on this point in the 

 Annelida ; but from our knowledge of the Arthropoda 

 we are inclined to the view that the preoral part of the 

 body does contain at least one pair of mesoblastic somites 

 (or their equivalent) the only real test of a segment 

 which are serially homologous with the other somites. 

 This is clearly shown in the embryo of Pcripatus, in 

 which the somites of the preoral region possess the rudi- 

 ments of a nephridium, and send extensions into the 

 preoral appendages called antennae ; just as the posterior 

 somites are prolonged into the legs. 



The prestomium is sometimes quite small and 

 inconspicuous ; it may however be large and 

 much elongated into a proboscis-like organ, as in 

 Nais lacustris and the Eclt/nnn'ilea. In some 

 cases it bears special sensory appendages called 

 tentacles and palps. It sometimes happens in 

 the Polyeliaeta that a certain number of body 

 segments are fused with the peristomium, forming 

 a secondary composite head. 



The central nervous system consists of two 

 nervous tracts, mainly ventral and called the 

 nerve cords. These are generally closely approxi- 

 mated in the middle ventral line over the greater 

 part of their course, but sometimes they arc 

 widely separated (some Polychaeta sedentaria)', 

 and in the Eclriuroidea the ventral parts of them 

 are fused to form a single ventral cord. But in 

 all forms, even in those in which there is com- 

 plete union between them behind, they separate 

 from one another in front, and embracing the 

 anterior part of the alimentary canal become con- 

 tinuous with one another on the dorsal side at 

 the front end of the body. The ventral portions of these cords are 

 almost always swollen at segmental intervals into the so-called 

 ganglia, from which the nerves generally proceed, and there is 

 universally a single or bilobed swelling at the point where they 

 are continuous with one another dorsal to the alimentary canal. This 

 dorsal swelling (or swellings) is called the cerebral ganglion (or ganglia), 

 or brain : the nerves which pass out from it (or rather which enter 

 it) are the sensory nerves of the anterior end of the body, and of 



FIG. 358. Anterior end 

 of Polyr/tinl!"* in"- 

 politanus, seen in 

 profile as a trans- 

 parency. The dis- 

 tinction between the 

 prestomium and 

 peristomium, and be- 

 tween the peristo- 

 mium and the next 

 segment is clearly 

 shown, b the mouth 

 (after Fraipont). 



