TO THE EXISTING ARTICl LATA. 45 



even protected by lateral lobes, we should certainly expect to find that modification which is 

 characteristic of the shell-bearing Phyllopods, but we are not at lil)erty to assume any close 

 analogy, since different families of animals, however nearly allied, do not exhibit the same 

 arrangements in detail, each particular organ not being similar in allied groups, but such 

 groups rather exhibiting general relations, and often showing marked differences in particular 

 points of structure. This law is illustrated in other ways in the case of the Trilobites, and 

 we cannot doubt its universality ; so that in giving a certain form to the feet in the restored 

 figure (See Plate VI, Figs 7, 8), I have done so rather intending to indicate what they may 

 have resembled, than with any idea of assuming their actual form. I merely assert that these 

 organs were soft, membranous, and fringed, adapted for locomotion in water, placed on the 

 abdominal portion of the body, and extending sidewise beneath the lateral lobes of the rings, 

 as shown in the ideal transverse section. (Fig. 7.) These feet were also indented, and thus 

 divided into several lobes at the open lower side, and each separate lobe was furnished at 

 the margin with small bristles serving as fins. The last and external lobe (c) was probably 

 longer, smaller, and more moveable, and reached to the termination of the protecting shell- 

 lobe (a), bearing a bladder-shaped gill {I/) on the inner side. The protecting lobes of the 

 feet of the P//////oj)oda were probably entirely absent in the case of the Trilobites, the hard 

 shell aff'ording sufficient protection, and the space beneath its lateral lobes not being large. 

 How far along the body the feet extended is a matter that 1 must leave undetermined, but I 

 am inclined to suppose that they may have reached the abdomen, as in Jpus, since the 

 caudal shield frequently exhibits the same impressions as the lateral lobes of the thorax, 

 and these impressions were no doubt connected with the existence of feet. The oblique 

 transverse furrow at each of the lateral lobes indicates perhaps that the foot was situated, 

 or perhaps partly attached behind it, at the broader part, which issues from the ring of the 

 axis, whilst the smaller anterior part of each lobe was adapted for articulation with the 

 preceding one, at least in those genera possessing the capacity of doubling themselves 

 into a ball, where there seems to be a deeper insertion at the spot where the lateral lobe 

 turns itself downwards, in proportion as the facility the animal had of doubling itself up is 

 greater. Since also the anterior oblique surface of the lateral lobes, which was pushed 

 beneath during the operation of doubling up, never reaches further than to this apparent 

 point of articulation, this circumstance renders still more probable the supposition of a 

 more intimate connexion of the lateral lobes with one another, from the axis up to this 

 very spot. 



It may also perhaps be a subject of investigation, whether the feet of Trilobites 

 resembled each other in shape and size, as in Branchipm, or whether the anterior were 

 different from the rest, and the posterior ones became gradually smaller, as in Jpm. Such 

 questions are no doubt difficult to answer, but still there are certain circumstances which 

 may help us in coming to a probable conclusion on the subject. 



And first of all I do not iinagine that any of those Trilobites capable of rolling them- 

 selves into a ball possessed the peculiar swimming apparatus observable in the first pair 

 of feet in Ajnis ; since this apparatus, consisting of long appendages projecting far beyond 

 the margin of the integuments, would seem to require special organs of retraction to 

 admit of being folded and concealed quickly and safely while the animal was rolling 

 itself up at tlie moment of danger, and this difficulty would exist even if they were not so 



