340 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



A. DIPLOBLASTICA. Embryo two-layered. 



PHYLUM I. CCELENTERATA (Hydra}. Radially symmetrical 

 Metazoa with body-wall composed of ectoderm, endoderm, and 

 mesogloea. There is a digestive cavity opening by a mouth, but 

 no body-cavity. 



B. TRIPLOBLASTICA (Ccelomata). Bilaterally symmetrical 

 Metazoa, usually with a body-cavity, and possessing mesoderm 

 in addition to ectoderm and endoderm. 



PHYLUM II. PLATYHELMIA (Distoma, Twnia). Unseg- 

 mented usually flattened worms, without lateral appendages or 

 distinct body-cavity. Nervous system of cerebral ganglia, nerve- 

 cord (not as double ventral cord), and nerve-plexus. Life-history 

 often complex. 



PHYLUM III. NEMATHELMIA (A scans). Elongated cylin- 

 drical unsegmented worms, usually with complete alimentary 

 canal, but no blood-system. Cuticle well developed ; ciliated 

 epithelium absent. Excretory organs as two unbrariched lateral 

 canals opening by a common anterior ventral pore. Sexes distinct. 

 Gullet surrounded by a nerve-ring from which two principal nerves 

 (lateral) run forwards and two (a dorsal and a ventral) backwards. 



PHYLUM IV. ANNELIDA. Elongated segmented worms, 

 with paired nephridia, a nerve-ring, and a ventral ganglionated 

 nerve-cord of double nature. Possess both blood-system and a 

 ccelom. 



Class 1. Chaetopoda (Lumbricus). Annelids possess- 

 ing setae, and with a well-developed coelom. 



Class 2. Hirudinea (Hirudo). Annelids devoid of 

 setae, with segmentation obscured by secondary annu- 

 lation, and with a reduced ccelom communicating with 

 the blood-system. 



PHYLUM V. ARTHROPOD A (Astacus). Segmented animals 

 possessing well-marked body-regions as a result of the differen- 

 tiation of segments. Lateral jointed appendages, and a nervous 

 system of the same type as in Phylum II., but with better- 

 developed ganglia, especially anteriorly. Blood-system largely 

 lacunar, replacing the coelom. 



