Class 2, Discophora. 



177 



narrower, and possess only eight eyes and very weak rudimentary jaws; the 

 cocoons are smooth, and are fastened to water plants. 



2. JRhynchobdellidw. With proboscis. Anterior organ of adhesion, 

 cup-shaped. Eggs large ; no metamoi-phosis. Fresh- water and marine. 



(a) Clepsine, a small, flattened leech, almost as hard as cartilage, which is 

 frequently found in fresh water. The eggs, enclosed in a veiy thin cocoon, and 

 the young ones, are earned about on the underside of the body of the parent, 

 which then seems to be hollowed out like a cup. 



(6) The Fish-leech (Piscicola), with cylindi-ical 

 body and bell-shaped sucker at both ends ; lives as a 

 parasite upon most species of marine fish. Nearly 

 related to this is the large Pontohdella muricata, with 

 large integumentary warts ; upon Skates in the North 

 Sea. 



Note. — A little worm, parasitic upon the Crayfish 

 (on the gills and elsewhere), Branchiobdella astaci, is 

 usually put with the Leeches. It approaches the Chaeto- 

 poda in some points, and by some authorities is counted 

 as one of this group. The body is cylindrical, the anterior 

 sucker indistinct ; it possesses two jaws and a gut without 

 diverticula. The conditions of the genitalia recall those 

 of the Oligochseta. 



Class 3. Onychophora. 



mBt» 



This division inolxides only the genus Peripatus, 

 -which may be regarded as a Ohsetopod adapted for 

 terrestrial life. 



In external appearance the Peripatus species are 

 most like caterpillars. The body is elongate and 

 cylindrical, the segments not externally demai-cated. 

 The skin is gi-anulai-, and delicately striated trans- 

 versely. At the anterior end there is a pair of ringed 

 tentacles (these appendages may be ringed also in the 

 Chsetopoda), and a pair of simple eyes of the kind 

 shown in Fig. 20, s. In the mouth there is a pair of jaw- 

 like masticatory organs. The rest of the body consists 

 of simUai- segments, each of which bears a paii- of 

 indistinctly jointed, stumpy limbs, ending in two 

 claws. The muscles are composed of smooth 

 muscle cells. The nervous system is chai'acterised 

 by separation of the ventral cords, which are joined by 

 many delicate transverse strands, whilst only feeble 

 swellings ai-e present in each segment. The alimen- 

 tary canal is a straight tube ; the anvis lies at the 

 posterior end of the body. The heart is dorsal, and is 

 a tube provided with lateral shts ; other vessels are 

 wanting. The respiratory organs consist of a 

 well-developed system of a i r - o a r r y i n g tubes, which 

 ramify in the body and open upon the sm-faoe in many 



delicate, in-egularly-distributed, respiratory apertm-es.J In most segments there 

 is a pair of segmental organs, similai- to those of other Aimelids : they 



Fig. 139. Peripatus, 

 from the dorsal aide. — 

 After BaKour. 



