SECT. XIII THE TRILOBITES 231 



number of limbs not too far from the mouth. It 

 was, no doubt, as in Apus, much reduced in other 

 parts of the body, in order not to occupy much 

 space. 



The ambulatory limb, if our homology is correct, 

 was the prolonged tip of the dorsal parapodium, and 

 thus homologous with the endopodite of other Crus- 

 tacea. The exopodite was the sensory cirrus ; and 

 here, no doubt in correspondence with the needs of 

 the animal, it retained its position close to the gills, 

 and its filiform shape ; it did not travel along the 

 prolonged dorsal branch of the parapodium, or 

 develop into a rowing flabellum as in Apus. The 

 modification of the gills into spirals, &c., is also very 

 easily explained on the grounds given above. The 

 animal required respiratory organs which afforded as 

 large a respiratory surface as possible while occupy- 

 ing the smallest possible space, such respiratory 

 organs being essential to the habit of rolling up. 



Before dismissing the subject of the form of the 

 Trilobite limbs, we wish to return for a moment to 

 Burmeister's assumption, that if the Trilobites were 

 related to Apus they must have possessed Phyllopo- 

 dan limbs. This, however, is by no means necessary. 

 What is generally known as the typical Phyllopodan 

 limb is but one of the ways in which the Annclidan 

 parapodium developed, the Trilobite ambulatory leg- 

 being another and quite independent modification 

 the parapodial type being visible in both. The 

 modifications arc due to adaptations to the different 

 manners of life adopted by the different groups. 



