PHYLOGENY 



The subphylum Arachnida includes a highly varied array of organisms 

 which have developed along lines quite divergent from the series just 

 discussed. It comprises the horseshoe crabs, scorpions, spiders, harvest 

 men, ticks, mites, and their allies. It is difficult to derive them from the 

 trilobites, but it may be possible. There is a tendency toward suppression 

 of segmentation, with only two body regions, the anterior prosoma and the 

 posterior opisthosoma being marked off. There are six pairs of appendages, 

 of which four pairs are walking legs. Unlike all other arthropods except 

 the trilobites, none of the appendages are specialized as jaws. Gnathobases 

 of the anterior limbs serve this function. 



The class Pycnogonida, the sea spiders, is a small and little-known 

 group which is usually placed with the Arachnida, simply because of 

 their superficial resemblance to spiders. Yet their morphology is utterly 

 different. Hedgpeth has studied the group thoroughly, and has concluded 

 that they are undoubtedly arthropods, but that they are so widely diver- 

 gent that they cannot reasonably be grouped with any of the other arthro- 

 pod types. Even less justification can be found for the common practice 

 of making the Tardigrada a class of arachnids. This is a small and little- 

 known group of minute, freshwater organisms which shows some relation- 

 ship to the Arthropoda, and has usually been treated as a class of the 

 Arachnida. But specialists in the field feel that it should be regarded as 

 an independent phylum of uncertain relationship to the Arthropoda. 



The Onychophora — A Unique Evolutionary Link. It was mentioned 

 above that the derivation of the Arthropoda from the Annelida is more 

 certain than is the derivation of any other phylum. This depends upon 

 the Onychophora, a group of about eighty species all of which are as- 

 signed to a single genus, Peripatus (Figure 49). This group of animals 

 shows a peculiar mixture of annelid and arthropod characters. Among the 

 annelid characters may be mentioned the general appearance of the or- 

 ganisms, for they look much like polychaetes in which the parapodia do 

 not bear chaetae. The cuticle is thin like that of annelids, and the muscles 

 of the body wall are continuous. The excretory organs of both annelids 

 and onychophorans are mesodermal tubules segmentally arranged (coe- 

 lomoducts), while those of the Arthropoda are usually entodermal or 

 ectodermal in origin. The reproductive ducts of the Onychophora are cili- 

 ated, but cilia are unknown among the Arthropoda. The eyes of the anne- 

 lids and onychophorans are simple, whereas those of the arthropods are 

 compound. On the other hand, in contrast to the annelids, the Ony- 

 chophora and the Arthropoda have jaws derived from appendages. The 

 coelom in each is much reduced and largely replaced by a hemocoel, 

 while the coelom of the annelids is highly developed. The circulatory sys- 

 tem of the Onychophora also resembles that of the Arthropoda rather than 

 that of the Annelida. Finally, the respiratory system of the Onychophora 

 consists of a set of tracheae, a characteristic known nowhere else but in 

 the Arthropoda. 



Because of this strange mixture of characters, the taxonomic position 

 of the Onychophora has always been a much vexed question. They were 

 originally treated as a class of the Annelida. But because of the presence 



146 



