222 NATURAL HISTORY. 



originated from hetnimetabolous ancestors. However it was introduced into the life-history of 

 the Insecta, this worm-like larval form is certainly their most important modification. As already 

 indicated, it enables each individual to play two distinct parts in the economy of nature, and it is by 

 its introduction alone that the internal parasitism, which is characteristic of so many families of 

 insects, is rendered possible. 



In some few insects, but much more strikingly in members of the classes Arachnida and 

 Crustacea, parasitism superinduces a metamorphosis of another kind, which is commonly known 

 as " retrograde metamorphosis," seeing that the adult parasite, instead of showing an advance upon 

 the structure of the newly-hatched young, exhibits a marked degradation of type. This curious and 

 interesting phenomenon is well shown in the numerous forms of parasitic Crustacea, such as the 

 Lerneadse and Rhizocephala, and especially in such parasitic Isopods as Entoniscus, &c., the larvae 

 and males of which display true Crustacean characters, while the parasitic females are mere egg-sacs, 

 which might very justifiably be taken for worms. The Cirripedia again exhibit another phase of 

 what must be termed retrograde metamorphosis. 



It will be seen, from the foregoing rapid sketch of the development of the Arthropoda, as also from 

 various statements contained in the preceding description of the classes and orders composing the 

 group, that whatever indications of alliances outside the group are presented by its members are all in 

 the direction of animals now included under the great division of the Yermes. In the general 

 description of the characters of the class Insecta, we took occasion to indicate that in former days the 

 Arthropoda and Vermes, as then understood, were regarded as forming a single great division of the 

 animal kingdom, the Annulosa, characterised by the ringed or segmented structure of the body 

 displayed by its typical members, and we must confess to a lingering doubt whether such a grouping 

 does not present a more philosophical idea of the relationships of these creatures than the one 

 now generally adopted. Under any circumstances, it is among the Vermes that we must seek the 

 nearest allies of the Arthropoda ; or, in other words, to adopt the views of the illustrious Darwin, 

 which, whether accepted as the expression of facts or not, must, as we have more than once 

 stated, furnish the guiding principles in inquiries of this nature, the ancestors from which they 

 were derived. 



It would seem, from the investigations of Mr. Moseley and others upon the curious genus 

 Peripatus, that the remarkable worm-like creatures forming it, which are so peculiar both in their 

 organisation and in their geographical distribution, represent the surviving progeny of organisms 

 directly uniting the Annelida (the highest class of Vermes) with the Myriopoda as we now know them. 

 If this be the case, one line of descent is very plain. The group of Chilognathous Myriopods (such as 

 Julus, &c.) would be easily derived from modified Peripati, and the transition from them to the 

 Chilopoda presents no difficulties, even from the consideration of existing forms. The production of 

 Myriopods must have taken place at a very early period of the world's geological history, as their 

 remains have been detected in Devonian rocks in America. The production of six-legged larvse by the 

 Juliform Myriopods, if not inherited from the Peripatoid ancestor, may have been superinduced as a 

 saving of material in the egg, and these larval forms lead directly to the truly ametabolous Thysanura, 

 among which Campodea is regarded by Sir John Lubbock as approximately representing the lowest 

 and earliest type of true insect, from which all the other multitudinous forms may have been derived 

 by descent with modification, the Hemimetabola retaining the direct mode of metamorphosis as above 

 described, starting from the Campodea-like larva and reaching the adult form by growth with 

 addition of parts; the Metabola proceeding from the latter by the superaddition of a vermiform larva 

 stage with its concomitant or resting pupa stages. In most cases the primitive larval form appears to 

 have become suppressed in the metabolous insects, although it is still retained, as above mentioned, 

 in Meloe, Sitaris, Stylops, and their allies. 



With regard to the Crustacea and Arachnida, we get no information from this assumed line of 

 descent, and the fact that the latter belong to the tracheate series of Arthropods renders the question 

 of their origin rather puzzling. It would appear, however, that the primitive larval form of the 

 Crustacea is the little creature described as a JVauplius (pp. 194-6), which is the first product of the 

 egg in the majority of the lower types of the class, while in the highest groups the young animal 

 is generally of the form originally described as a distinct genus under the name of Zoea (pp. 194-6), 



