326 WILLIAM A. HASWELL. 



and that an imperfect one, is developed, namely, that at the 

 commencement of the tail region. 



In both families a head is present, and is clearly marked 

 off from the rest. Its composition, however, does not in any 

 way correspond in the two cases. lu the Polygordiidse it 

 consists, as in most Chaetopods, of a prostomium lodging the 

 brain, and of a peristomium, on the ventral surface of which 

 the mouth opens. In the Histriobdellidse it is undivided, 

 and the mouth opens far forwards, near its anterior extre- 

 mity in front of the brain. The presence on the head of the 

 remarkable retractile anterior limbs is highly characteristic 

 of the Histriobdellidae, as is also the presence further back 

 of the retractile claspers in the male. The highly developed 

 posterior limbs with their glands are also special structures. 



One of the most characteristic features of the structure of 

 the Histriobdellidae is the presence of the elaborate jaw 

 apparatus, which is not represented in the Polygordiid^, 

 though the muscular sac appended to the oesophagus in the 

 members of the latter family may correspond to the sac in 

 which the jaws are lodged in the Histriobdellidge. 



A blood-vascular system is fairly well developed in the 

 Polygordiid^, but is not present in the Histriobdellidse. 

 The nervous system is much more highly developed in the 

 latter than in the former, and the venti'al nerve-cord takes 

 the form of a chain of ganglia metamerically arranged, 

 whereas in the Polygordiidae there is no trace of ganglia. 



The reproductive organs of the Polygordiidae are of a 

 generalised character, and are constructed on the same 

 general plan as those of the Polychseta. In the Histriobdel- 

 lidee they are highly specialised. On the whole it appears to 

 me that the relationship between the Histriobdelhdae and the 

 Polygordiidae is extremely remote, and not such as to justify 

 their inclusion in the same class. 



A comparison with Dinophilus reveals certain common 

 features not shared with the Polygordiidae. In both Dino- 

 philus and the Histriobdellidae the animal consists of a dis- 

 tinct head, and a trunk consisting of a small number of 



