CONCLUDING REMARKS ON THE ARTHROPODA. 223 



For a long time it was supposed that this constituted a positive distinction between the lower and 

 higher Crustacea (some intermediate forms, Edriophthalnria, showing no larval forms at all), but the 

 researches of Fritz Miiller proved that in certain Shrimps (Penceus, pp. 194-6) the Zoea-iorm was 

 preceded by a Nauplius-form, thus furnishing analogous evidence to that existing in the case of insects. 

 of the existence among Crustacea of a primitive and a superadded larva (Nauplius and Zoed), of 

 which the former had in many cases become suppressed. 



If we consider the Crustacea to follow the same principle in their development as the insecta, 

 we must regard the Nauplius, or some Naupliform creature, as the primitive form of the class from 

 which all the rest have proceeded by descent with continual additions and modifications of parts ; 

 but it is hardly possible with our present knowledge to indicate the particular type of the Vermes 

 from which, or from the larva of which, the primitive Nauplius could have originated. But the 

 interesting fact becomes perfectly plain that as the Peripatus could have nothing to do with the 

 origin of the Crustacea, the evolution of the Arthropod type must have taken place along, at least, 

 two lines of descent of different origins, a view which was adopted by Prof. Balfour in his very 

 valuable work on "Comparative Embryology." It is possible that the starting-point of the 

 Crustacean line was from some organism pertaining or approximating to the group of creatures 

 now known as Rotifera. 



The Arachnida, through the curious little Tardigrada, which are generally considered to have 

 Rotatorian affinities, may have originated from the same point as the Crustacea, but if so they must 

 have diverged at an exceedingly early stage of the evolution and formed a branch of their own, 

 gradually acquiring those characters which bring them apparently into affinity with the Insects 

 and Myriopods. The character of the respiratory organs, which has been adopted for the division of 

 the Arthropoda into two main groups of Tracheata and Crustacea is evidently of no consequence 

 in connection with this question of descent, seeing that it is quite clear, from the analogy of Peripatus 

 and the Earth Worms, that throughout the Annulosa the priuciple on which the originally aquatic 

 forms are adapted for aerial respiration consists in the substitution for the primitively exposed 

 branchial organs of concealed cavitary organs, the arrangements by which the blood is brought in 

 contact with the respiratory medium being strictly homologous in both cases. The passage upward 

 through the Mites to the Spiders and Scorpions may then easily be conceived. The parasitic forms, 

 such as the Linguatulina, originated by a process of retrograde metamorphosis ; while the singular 

 group of the Pantopoda, with their remarkable larvae, would seem to have remained from an early 

 period at a very low stage of development.* 



Another group, which we have here placed with the Crustacea, is regarded by some naturalists 

 as belonging truly to the Arachnida. This is the order Xiphosura, including of living forms only the 

 few species of King Crabs or Horseshoe Crabs, the structural relations of which to the Scorpions 

 would seem to be very close, and certainly raise a difficult problem, and one which is rendered still 

 more interesting by the fact that, according to the researches of Dr. Jiiles Barrois, a Limuloid or 

 King Crab-like stage occurs in the development within the egg of certain true Spiders. For the 

 present this and many other such questions must, however, remain open. In all biological problems 

 relating to the past developmental history of the organic world, we must for a long time yet 

 expect to come continually upon obscure and puzzling points, which only a more extended knowledge 

 of minute details can clear up. 



* Professor Balfour (Comp. Embryol., vol. i., p. 448) says of the Pycnogonida : " The six-legged larva has none of the 

 characteristic features of the Nauplius, except the possession cf the same number of appendages ; " but he places the group 

 .among those cf doubtful affinities. 



W. S. DALLAS. 

 HENRY WOODWARD. 



