— 20 — 



h' with a mouth and alimentary canal ; no protrusible 

 proboscis ; ova not liberated into the body cavity 

 i'' nervous system composed of an anterior ring and 

 six longitudinal truni^s, the two largest dorsal and 

 ventral ; lateral lines and oral papillae, hooks 

 or spines present ; genital organs single or paired 

 tubes 



Nematoda. 



r nervous system consisting of three cords close 

 together in the mid- ventral line, v^ithin the muscular 

 layer ; muscular system of a single layer of longi- 

 tudinal fibers ; no lateral lines nor oral papillae ; 

 genital organs metamerically repeated masses of cells 



Nematomorpha . 

 no mouth nor alimentary canal ; with a protrusible 

 proboscis furnished with hooks; ova liberated into 

 the body cavity, into which the oviducts open 



Acanthocephala. 



e' with a vascular system and usually also a respiratory system; 

 with a true cœlome separated off from, but distinct from, 

 the enteric cavity ; digestion (except in a few parasitic forms) 

 always extracellular ; symmetry quadrilateral ; development 

 indirect through a larval stage, or direct 



r no true skeleton forming an integral part of the body ; 

 development through a trochosphere larva, or direct 

 g' body externally unsegmented 



h' worm like, with a single ventral nerve cord ; appen- 

 dages confined to buccal tentacles, or absent 

 i' nervous system in continuity with the ectoderm; 

 alimentary canal almost or quite straight, the anus 

 terminal; pharynx lined with chitinous teeth; no 

 buccal tentacles ; no canalicular vascular system 



Priapuloidea. 



V nervous system not in continuity with the ectoderm ; 

 alimentary canal coiled and bent upon itself, the 

 anus being near the anterior end of the body ; 

 peristome with a circlet or double horseshoe of 

 tentacles ; no teeth; a closed vascular system 



Sipiinculoidea. 



h"" form very varied ; nervous system of paired cords, 

 loops or commissures ; usually with an excreted 

 calcareous shell in the shape of a straight or coiled 

 tube, two more or less similar valves, or one or 



several plates Mollusca. 



g" body externally segmented, or at least with segmen- 

 tally arranged appendages 



h' external segmentation agreeing with the internal ; 

 development through a trochosphere (except in the 

 Oligochaeta) 



i' no setee nor parapodia Archiannelida. 



V with setae ; with or without parapodia 



y with parapodia ; usually with a more or less definite head bearing 

 appendages, and chitinous jaws Polychaeta. 



y no parapodia, tentacles, cirri, nor jaws 



k' segmentation always strongly marked ; development direct 



Oligochaeta. 



