Introduction to Animal Morphology. 175 



formed before their expulsion from the mother, 3. Polysto- 

 midee — with an anterior small, and six posterior larger suckers 

 on the flattened margin ; hooks 2 (Polystoma), or 8 (Erpo- 

 cotyle). Two of these have been found as human parasites, 

 Polystoma pinguicola (ovary, Tretitler), and Tetrastoma renale 

 (kidney, Delle Chiaje). 4. Udonellidae — leech-like ; minute ; 

 front of body with or without lateral suckers ; intestine 

 simple ; hinder suckers sessile, with no hooks, often found 

 on Lerneee. 5. Mj-zostomidae — aberrant, monoecious forms; 

 ecto-parasitic on Comatula, and with possible' affinities to 

 Ch^topoda, to Tardigrada, or Crustacea ; discoidal, soft, 

 surrounded by cilia, with latero-abdominal suckers, and hooks 

 like those of other trematodes ; a protrusible proboscis ; a 

 wide stomach ; a narrow dendriform intestine, and afi anus ! 

 They have holoblastic ova, and ciliated, cylindroid larvae, 

 which develop two pairs of processes like rudimental feet ; 

 these increase to five pair, and resemble the parapodia of 

 Chsetopods ; in M. cirrhiferum, each bears a bristle ; the 

 nerve system has a pharyngeal commissure and a few ventral 

 gangliform swellings ; the male sex-organs are double, and 

 open with the intestine ; the testes are branched pouches ; 

 the female organs open separately {Loven) or with the intes- 

 tine {Se7?iper). 6. Tristomidae — hinder sucker large, radiated, 

 with or without a stalk ; in front are two round or long lateral 

 suckers ; intestine branched, with sex openings separate on 

 the left side. Cyclatella has a crown of tentacles ; Tristoma 

 has hooks on the hinder sucker, ex. Callicotyle, Epibdella ; 

 some have eye-specks. 



Sub-order 2. Digen^a {Van Beneden) — eggs small, nume- 

 rous ; development metagenetic ; the e^gg emits a variably 

 shaped larva, with or without cilia, sometimes with a touch 

 papilla and two lateral plates anteriorly ; within this larva 

 arises a cylindrical, tailed body, which becomes free, and 

 may possess a mouth, pharynx, and water-vascular system,' . 

 opening outwards atid into the body cavity. This form sheds 

 its outer ciliated layer, and is known as a Redia. Sometimes 

 the water-vascular system extends into the tail, and there 

 may be a pulsating vesicle there. In other cases, the larva 

 develops within itself a rounded, or worm-like, tail-less, 



