THE PHYLUM ANNELIDA 



remainder of the vascular system is "closed." Reproductive habits vary; in 

 manv small aquatic forms sexually mature individuals have never been recog- 

 nized, and reproduction occurs exclusively by a process of transverse fission 

 (cf. rhabdocoel turbellarians, p. 325). One entire family of aquatic oli- 

 gochaetes, the Branchiobdcllidae (Hg. 14.16), have become adapted to an 

 ectocommensal or ectoparasitic existence. These minute worms pass their 

 complete life cycles attached among the gills and swimmerets of the fresh- 

 water crayfish. In correlation with this way of life, they show several modifi- 

 cations, notablv an (^xpanded posterior sucker for adhesion to the host. 



The Class Archiannelida 



The Archiannelida are small marine worms which as adults resemble in 

 manv wavs the juvenile stages of polychaetes. As previously indicated, the 

 archiannelids may be considered as either primarily simple, generalized forms 

 or as secondarily simplified types. The varying characteristics of members of 

 this class indicate that they may not constitute a taxonomically homogeneous 

 group. A representative example, Polygordius, shows externally only indistinct 

 evidences of segmentation (Fig. 14.17). Internally, however, it possesses the 

 metamerism and other features characteristic of the phylum. The develop- 

 mental sequence of stages in Polyaordius, involving a typical trochophore larva 

 with gradual metamorphosis into an elongate, segmented adult, is so simple 

 and straightforward that it has been extensively studied as typical of the 

 primitive development of annelids. 



The Class Hirudinea 



The Hirudinea, or leeches, are predominantly fresh-water annelids, al- 

 though some are marine and others have become adapted to life in a moist 

 terrestrial environment. Most leeches are temporarily or permanently ecto- 

 parasitic, feeding on the blood of vertebrate hosts. Some fresh-water species 

 are known which are not parasitic but feed instead on snails and other small 

 aquatic organisms. The typical annelid structure is the fundamental plan of 

 organization in leeches, although in correlation with the parasitic habit of 

 the group considerable modification and specialization have occurred (Fig. 

 14.18). The body has 32 segments, each subdivided externally into three or 

 more rings, and is conspicuously flattened dorsoventrally. There are no 

 parapodia and no setae. Some leeches have horny jaws with which incisions 

 are made in the skin of the host; others have an unarmed probo.scis. The 

 mouth lies in the center of a small oral sucker, used in feeding and in 

 locomotion; a larger sucker at the posterior end holds the leech securely upon 

 its host or elsewhere. In locomotion looping movements are executed, the 



417 



