iv VERMESFORM OF BODY AND ORGANISATION 189 



blood-vessels. In the Aphroditidce they become broad dorsal scales 

 (elytra). In some cases (e.g. Capitellidce, Glyceridce) the dorsal para- 

 podia may carry, besides the typical cirrus or the cirrus transformed 

 into a lateral organ, a gill, which, in contradistinction to the gill which 

 arises from the cirrus, is called a lymph gill; it is provided with haemo- 

 lymph by continuations of the body cavity (in the absence of a blood- 

 vascular system). The cirri may stand at the base of the parapodia, 

 or may even move away from them. They may even be retained in 

 those cases where the parapodia disappear. 



The head of the Polychceta is characterised by special appendages, 

 the front ones being called feelers, the back ones feeler-cirri. They 

 never stand on parapodia, which are just as constantly wanting in the 

 true oral segment as are setae. In most of the delicate tubicolous 

 Polychceta a reduction of the parapodial cirri or parapodial gills goes 

 hand in hand with the reduction of the parapodia on the trunk ; they 

 are retained only in the anterior segments. In the Serpulidce all the 

 parapodial appendages are reduced, and therefore the head append- 

 ages are transformed into greatly developed tentacle gills which often 

 form a stately crown. In Sternaspis on each side of the anus there 

 is a tuft of gills. 



In the Oligochceta, Archiannelida, Echiuridce, and some of the Capitel- 

 lidce, not only the parapodia but their appendages (cirri, gills) are 

 wanting. Only, Alma nilotica, a very insufficiently known Oligochcete 

 which is found in muddy ditches in Egypt, carries dorsal gills on the 

 hinder part of its body. In all these divisions the head appendages 

 are also wanting. Only the Archiannelida (Polygordius, Protodrilus) 

 possess two feelers at the extreme front of the head. In the Echiuridce 

 the head is produced in front of and over the mouth into a long pro- 

 cess provided with a longitudinal furrow or channel on the ventral 

 side (proboscis, prostomium); this in Bonellia is forked at its end. 



The inner segmentation is reflected outwardly in most Chcetopoda, 

 not only by the regular repetition of the setae (and the parapodia of 

 the Polychceta), but also by an outer division of the body into rings, 

 which is caused by the occurrence of more or less distinct regularly- 

 repeated constrictions. These constrictions are generally found 

 between 2 consecutive segments, and thus the rings, in number 

 and position, answer to the real segments. It only seldom occurs 

 that each segment is again ringed. In many of the lower Oligochceta, 

 indeed in the Archiannelida and some of the Echiuridce, no distinct 

 mark of rings or segmentation is recognisable on the integument. 



The body of the Chcetopoda is outwardly either homonomously 

 segmented, i.e. all consecutive segments of the trunk are alike, or 

 heteronomously segmented, when the segments in different regions 

 are differently developed, both as concerns their outer shape and their 

 provision with various setae, parapodia, cirri, gills, etc. We can in the 

 latter case distinguish different regions of the body (e.g. thoracic region, 

 branchial region, abdominal region, etc.) The integument of the Chceto- 



