TO THE EXISTING ARTICULATA. 51 



Fischer's figure may be considered as another. The first six rings certainly here appear to 

 be much more peculiar than the three next succeeding ones ; but, considering the other 

 points of identity in structure between this species and the North American one, we are 

 probably justified in assuming that they must have had an equal numerical proportion, so far 

 as the thorax is concerned. According to this view three, or perhaps six rings would come 

 to the abdomen, and this proportion would correspond with the prevalent type both of the 

 Trilobites and Phyllojioda. A separation of both divisions of the body in six and six, or in 

 six and nine rings, is, however, also conceivable, and indeed easily to be accounted for by 

 the analogy oi Apus and also of the species oi Pkacops referred to the second division. 



SECTION XXVII. 



Having adduced so many analogies and homologies of structure between the Trilobites 

 and the Pliyllopoih, we may be permitted to assume some similarity also in the habits of the 

 two groups, and I now propose to add some remarks on this subject. 



The PhyUopoda live in stagnant fresh waters, especially in ditches, pools, or puddles, 

 which are very rapidly produced after rain in the early part of the year, and last only till 

 the middle of the summer, when they become dried up. During this period the animals 

 of the tribe we are describing are usually seen in numerous companies swimming about in 

 the water at various depths, the species of BrancJiipus being most frequently close beneath 

 the surface of the water. In swimming they turn their back downwards, their abdomen 

 being upwards, so that the feet touch the surface of the water, and accordingly Branckipus, 

 has its eyes in the position in which I have represented it in Table VI, Fig. 3, and not 

 proceeding from the head rectangularly. This position of the eyes enables the animal to 

 look both upwards and downwards. Apus, on the other hand, which has immoveable eyes, 

 can only look downwards while swimming on its back, and it must turn itself if it wishes to 

 look upwards. But this is quite natural, since in each case the animal, when in its usual 

 position, and close beneath the surface of the water, can only have its enemies below, and 

 therefore only needs to be secured against surprise in that dii-ection. These creatures are, 

 however, not much exposed to attack. Their prey, which consists of other little animals 

 living in the water, they obtain during their constant swimming motion, and it is brought to 

 the mouth by the motion of the water. Owing to this, the region and cavity of the mouth in 

 many of these animals, when preserved in spirit of wine, are frequently entirely covered or 

 filled up with extraneous substances. The PhyUopoda are never at rest, partially because they 

 are entirely deficient in organs by which they could keep firm hold of anything, partially also 

 because their motion of swimming produces at the same time the motion of the respiratory 

 organs, which being independent of the will of the animal does not cease. I have not yet 

 had an opportunity of observing Limnadia and Estheria in a living state, but both genera 

 undoubtedly exist in the same manner ; I am not, however, aware whether they swim on 

 their back. Considering the afiinities of the Trilobites with the PhyUopoda, I cannot doubt 

 for a moment that their habits during life and their mode of existence were similar, and I 

 therefore conclude — 



