32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



(Ch), or tritocerebral appendages, the pedipalps (Pdp), which are the 

 mandibular appendages of other groups, and four pairs of legs (L), 

 which are represented by the two maxillae and the first two pairs of 

 maxillipeds in the Crustacea. Antennae are lacking in all adult arach- 

 nids, but some writers (Croneberg, 1880, Jaworowski, 1891) have re- 

 ported the presence of antennal rudiments in the embryos of certain 

 species (fig. 22 B, Ant). The comparative lack of specialization in the 

 arachnid limbs suggests that the Arachnida are an ancient group of 

 arthropods having little direct relationship to other forms, except to the 

 Xiphosura and possibly to the extinct eurypterids. In the Solpugida. 

 the cephalothoracic region is divided into an anterior cephalic part 

 carrying the eyes, the mouth, the chelicerae, the pedipalps, and the 

 first pair of legs, and into a posterior thoracic |)art carrying the second, 

 third, and fourth pairs of legs. The division between these two body 

 parts, as compared with insects, falls between the first and second 

 maxillary segments, and the parts, therefore, are in no way compar- 

 able with the insect head and thorax. In the ticks (Ixodoidea), the 

 head-like structure known as the capitulum is said to bear only the 

 chelicerae and the pedipalps. In its composition it is thus equivalent 

 to the protocephalon with only the first gnathal segment added. 



Cephalization in the Arthropoda, then, apparently has progressed 

 from the prostomial stage (archicephalon) to the formation of a 

 protocephalon, from a protocephalon to the usual definitive head, or 

 telocephalon, and finally to the union of head and body regions in a 

 cephalothorax. The archicephalic stage is to be inferred from the 

 evident derivation of the arthropods from an annelid-like ancestor 

 having the prostomium as the only defined head. The protocephalic 

 stage is shown in the development of all arthropod embryos, and is 

 retained in the decapods and related crustaceans, where the carapace 

 is a gnatho-thoracic structure. The telocephalic stage exhibits a 

 progressive evolution in phyllopods, amphipods, and isopods by the 

 addition of one, two and three, four or five segments to the protoce- 

 phalon ; in insects and myriapods it has reached a standardized con- 

 dition in which the head is composed of six segments and the pro- 

 stomium. The cephalothoracic stage is characteristic of the Xiphosura 

 and Arachnida. in which the segments of all the fully developed 

 appendages are united, and combined with the prostomium. 



A study of the head alone does not furnish a sufficient basis for a 

 discussion of the inter-relationships of the various arthropod groui)s. 

 but it must be recognized that the facts here given, and others to be 

 descril)ed in this paper have an important bearing on the subject. 



