MVRIOPODA. 



323 



ANNELID CHARACTERISTICS. 



Segmentally arranged nephridia 



as in Chaetopods. 

 The muscular ensheathing of the 



body. 



The cilia in the genital ducts. 

 Less important are the stump-like 



hollow legs and the simple 



eyes. 



ARTHROPOD AND TRACHEATE 

 CHARACTERISTICS. 



The presence of tracheae. 



The nature of the heart (a tube 

 with paired ostia communicat- 

 ing with a pericardium) and 

 the lacunar circulation. 



The modification of appendages 

 as mouth organs. 



The form of the salivary glands. 



The smallness of the genuine 

 coelom ; the cavity of the 

 body is haemoccelic. 



The Onychophora differ from other Tracheata Antennata in the 

 simplicity and diffuseness of the tracheae, in having only one pair of 

 jaws, in the absence of external segmentation, in the nature of the 

 body wall, and so forth. 



The ladder-like character of the ventral nervous system (cf. primitive 

 Molluscs, Phyllopod Crustaceans, and Nemerteans) is probably primi- 

 tive. That salivary glands and genital ducts are homologous with 

 nephridia is a fact of much morphological interest. It is possible that 

 the slime glands are modifications of crural glands, and that the latter 

 are homologous with the parapodial glands of some Annelids. It is 

 not certain that the antennae, jaws, and oral papillae of Peripatus 

 precisely correspond to the antennae, mandibles, and first maxillae of 

 Insects. 



Our general conclusion is that Peripatus is an archaic type, a sur- 

 vivor of forms which were ancestral to Tracheata and closely related to 

 Annelids. 



Second Class of Tracheata Antennata. MYRIOPODA. 

 Centipedes and Millipedes 



The centipedes and millipedes, which are grouped 

 together in the class Myriopoda, are usually elongated, 

 somewhat vermiform animals, with a distinct head and a 

 very uniform segmented trunk. The head bears eyes 

 (groups of eye-spots, not compound eyes like those of 

 insects, except in Scutigera), jointed antennae, and two or 

 three pairs of jaws. The segments of the trunk bear six- or 

 seven-jointed legs with terminal claws, very similar through- 

 out. The nervous system, the tracheae, the heart, the ex- 

 cretory tubules, etc., are like those of Insects. It cannot 



