THE ARTHROPODA. 427 



type which is universal among the Arthropoda — a dorsal vessel with 

 lateral pairs of ostia. The development of Peripatus also, for a 

 knowledge of which we must start from the Jsew Zealand form 

 which unfortunately is too little understood, is linked on to that of 

 other Arthropoda,* We have here in the first place an egg rich in 

 yolk, with superficial cleavage. The characters in which Peripatus 

 stands opposed to the Arthropoda are the position and constitution of 

 the extremities which are not actually jointed (we leave out of account 

 here the Tardigrada and Pentastomum, the relationships of which are 

 uncertain), and especially the structure of the eyes, which must be 

 regarded as an inheritance from Annelidan ancestors that was lost in 

 other Arthropoda, and replaced by the ommateal eyes (ocelli and 

 facet-eyes). 



After what has been said above, we seem to be justified in assuming 

 for all Arthropoda (Peripatus included) a common origin from the 

 Annelidan stock. In giving the name Protostraca to the hypothetical 

 racial form of the Arthropoda which proceeded from the Annelida, 

 and for which very primitive characters must be assumed if it is to 

 serve as the starting-point of all known classes of Arthropods, the 

 fact is expressed that the Crustacea in certain features of their 

 organisation, especially in structure of their limbs, which can be 

 traced back to the biramose form of parapodium, have preserved, in 

 consequence of their retention of a pelagic manner of life, primitive 

 characters. On the other hand, the form of extremities found in 

 Peripatus (and partly retained in the Palaeostracaf) must be con- 

 sidered as a secondary adaptation to a terrestrial existence. 



Starting from the Protostraca, according to the present condition 

 of our knowledge, we may, as has been already remarked, assume 

 three great series of development of the Arthropodan stock, by 

 the side of which a number of smaller independent branches 

 have been retained. One of these series leads through the hypo- 

 thetical primitive Phyllopod to the Crustacea; the second through 

 the Palaeostraca to the Arachnida : the third through forms resem- 

 bling Peripatus to the Myriopoda and the Insecta. The Pantopoda 

 and the Tardigrada must probably be regarded as smaller, independent 

 branches of the Arthropodan stock. 



If we try briefly to enumerate the general characters in winch the 

 Arthropoda are to be distinguished from the Annelida, we must point 

 first to the great development of the cuticular integument and the 



* [See footnotes, pp. 165 and 216.- Ed.] 

 t [See Vol. Li., p. 333, footnote.— En.] 



