Introduction to Animal Morphology. 175 



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

 midae 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, Trcutlcr\ and Tetrastoma renale 

 (kidney, Ddlc Chiajc]. 4. Udonellidae leech-like ; minute ; 

 front of body with or without lateral suckers ; intestine 

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

 on Lernece. 5. Myzostomidae aberrant, monoecious forms; 

 ecto-parasitic on Comatula, and with possible 1 affinities to 

 Chcetopoda, 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 an 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 

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

 m has a pharyngeal commissure and a few ventral 

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



vith the intestine ; the testes are branched pouches ; 

 th.- female organs open separately (Loven) or with the intes- 

 tine (Simper). 6. Tristomidae hinder sucker large, radiated, 

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

 SIR -kers ; 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. Di^ena,-a (\\jn Bcncdcti) eggs small, nume- 

 rous; development metagenetic; the egg emits a variably 



1 larva, witli or without cilia, sometimes with a touch 

 papilla and two lateral plates anteriorly; within this larva 



a cylindrical, tailed body, which heroines free, and 



;ih, pharynx, and water-vascular System, 



::g outwards and into UK body cari'/v. This form sheds 



.iiatrd layer, ami i Keclia. Sunn-times 



is into the tail, ami there 



le there. In other cases, the larva 



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



