480 INVEBTEBRATB MORPHOLOGY. 



Affinities of the Protracheata. — Peripatus is, a highly-suggestive form 

 on account of possessing both Annelidan and Tracheate characteristics, so 

 that it lias been generally regarded as indicating a descent of the Tracheate 

 forms from the Annelids. Its Annelidan features are, first, the presence of 

 a distinct dermal muscular system; second, the occurrence of crural glands 

 which seem to be homologues of the glands which secrete the setse in the 

 Annelida; third, the possession of nephridia corresponding closely to those 

 of the Annelids; and fourth, the structure of the eyes. On the other 

 hand, its Tracheate aflBnities are shown by the claw-tipped feet, by the 

 adaptation of the feet (mandibles) for masticatory purposes, by the ten- 

 dency towards a concentration of the anterior segments to form a head, 

 and by the occurrence of tracheae. Both these sets of features are highly 

 important, and, taken with the wide distribution of Peripatus, point 

 strongly to its being the representative of a connecting link between Tra- 

 cheates and Annelida, a phylogeny which may be considered more in de- 

 tail at the close of this chapter. 



II. Class Myriapoda. 



The Myriapoda possess a distinct head composed of a num- 

 ber of fused segments and followed by a distinctly-segmented 

 body formed of a varying number of segments, all of which 

 are more or less similar, there being no differentiation of a 

 thorax and abdomen. A single pair of appendages as a rule 

 is borne by each segment, with the exception, in some cases, 

 of the last. The most anterior pair are usually long multi- 

 articulate antennae, the second pair mandibles, and the third 

 and fourth, or the third alone, are modified to form maxillse; 

 the succeeding pairs, with one or two exceptions, are ambula- 

 tory, and are jointed and tipped by a claw. 



The chitinous cuticle is generally thick, and consequently 

 no definite system of dermal muscles is developed, a number 

 of separate muscles occurring in each segment for moving the 

 appendages and the various segments upon oue another. 

 Glands of various kinds opening upon the surface of the body 

 occur, the most important being glands or protrusible gland- 

 ular sacs situated upon the basal joints of a number of the 

 appendages and apparently homologous with the crural 

 glands of Peripatus. 



The heart is in all forms very long, extending through the 

 entire length of the body behind the head, and possessing 



