294 THE INVERTEBRATA 



oxygen : when the oxygen content of the water in general falls a greater 

 length of the worm is protruded and its movements become more 

 vigorous. A great deal of detritus passes through its alimentary canal 

 so that Tubifex plays the same sort of part in fresh water that the 

 earthworms play on land. 



Lumbriculus resembles Tubifex superficially but has only eight 

 chaetae in a segment, placed as in Lumbricus; chaetae double pointed; 

 not often met with in sexual state but reproduces habitually by 

 breaking up into pieces each of which regenerates the missing 

 segments. 



In this worm the primitive nature of the blood system is well seen 

 (Fig. 203). At the posterior end there is a continuous sinus round the 

 gut, in the middle region this becomes resolved into a dense plexus 

 of capillaries and at the anterior end there is the beginning of a seg- 

 mental arrangement. 



Class ARCHIANNELIDA 



Small marine annelids with simplified structure, parapodia and chaetae 

 being usually absent. 



This group was founded to receive two genera, Polygordius and 

 Protodrilus, which were formerly considered to be primitive forms 

 from which the larger groups of annelids might be derived. From 

 time to time other genera have been included which show some, but 

 not all, of the characters which distinguish the original genera. The 

 series of diagnoses of the best known genera given below starts with 

 Polygordius and works back to forms which come very close to the 

 Chaetopoda. There can be little doubt that the Archiannelida are 

 derived from this latter group by the loss of some of its distinctive 

 features (e.g. parapodia and chaetae), and retention of juvenile 

 characters (ciliation and connectionof nervous system with epidermis). 

 These changes are also found within the limits of the Polychaeta, and 

 if it was not that other characters link up its members the group might 

 well be considered as a family of polychaets. Dinophilus comes late 

 in the series because, though evidently related, it does stand rather 

 apart. It has a superficial resemblance to a small turbellarian 

 enhanced by the great reduction of the coelom. 



Polygordius (Fig. 198 B) with elongated cylindrical body, head 

 with two tentacles and ciliated pits; without parapodia or chaetae; 

 with segments of the coelom separated by septa with a pair of seg- 

 mental organs opening into each by nephrostomes; with longitudinal 

 muscles in four quadrants, the circular muscles being usually absent; 

 with a reducedvascular system and nerve cords lying in the epidermis ; 

 with a trochosphere larva, Fig. 198 A. 



