Fur Darwin. 261 



maggots of the Diptera or Hymenoptera, the larvae of the Coleoptera, or the cater- 

 pillars of the Lepidoptera, still less any bearing even a distant resemblance to 

 the quiescent pupae of these animals. The pupae, indeed, cannot at all be regarded 

 as members of an original developmental series, the individual stages of which 

 represent permanent ancestral states, for an animal like the mouthless and footless 

 pupa of the Silkworm, enclosed by a thick cocoon, can never have formed the 

 final, sexually mature state of an Arthropod. 



In the development of the Insecta we never see new segments added to 

 those already present in the youngest larvae, but we do see segments which were 

 distinct in the larva afterwards become fused together or disappear. Considering 

 the parallelism which prevails throughout organic nature between palaeontological 

 and embryonic development, it is therefore improbable that the oldest Insects 

 should have possessed fewer segments than some of their descendants. But the 

 larvae of the Coleoptera, Lepidoptera, &c., never have more than nine abdominal 

 segments, it is therefore not probable that they represent the original young form 

 of the oldest Insects, and that the Orthoptera, with an abdomen of eleven seg- 

 ments, should have been subsequently developed from them. 



Taking into consideration on the one hand these difficulties, and on the 

 other the arguments which indicate the Orthoptera as the order most nearly 

 approaching the primitive form, it is my opinion that the "incomplete meta- 

 morphosis" of the Orthoptera is the primitive one, inherited from the original 

 parents of all Insects, and the "complete metamorphosis" of the Coleoptera, 

 Diptera, &c., a subsequently acquired one. 



Zu S. 258: 



I will only give, as an example, the probable history of the production of a 

 single group of Crustacea, and indeed of the most abnormal of all, the Rhizo- 

 cephala, which in the sexually mature state differ so enormously even from their 

 nearest allies, the Cirripedia, and from their peculiar mode of nourishment stand 

 quite alone in the entire animal kingdom. 



I must preface this with a few words upon the homology of the roots of 

 the Rhizocephala, i.e. the tubules which penetrate from its point of adhesion into 

 the body of the host, ramify amongst the viscera of the latter, and terminate in 

 caecal branchlets. In the pupae of the Rhizocephala (fig. 58) the foremost limbs 

 ("prehensile antennae") bear, on each of the two terminal joints, a tongue-like, thin- 

 skinned appendage, in which we may generally observe a few small strongly 

 refractive granules, like those seen in the roots of the adult animal. I have there- 

 fore supposed these appendages to be the rudiments of the future roots. A per- 

 fectly similar appendage, "a most delicate tube or ribbon", was found by Darwin 

 in free-swimming pupae of Lepas australis on the last joints of the "prehensile 

 antennae". From the perfect accordance in their entire structure shown by the 

 pupae of the Rhizocephala and Cirripedia, there can be no doubt that the append- 

 ages of Sacculina and Lepas, which are so like each other and spring from 

 the same spot, are homologous structures. 



Now in three species of Lepas, in Dichelaspis Wa'rwickii and in Scalpellwn 

 Peronii, Darwin saw, on tearing recently-affixed animals from their point or 

 support, that a long narrow band issued from the same point of the antennae; 



