THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 18*i 



to the exterior, and this would correspond to the nephridial canal 

 of the trunk -segments. In fact, there is such close agreement 

 between the antennae and limbs as rudiments, that it is difficult 

 to believe that they are essentially different structures. We should 

 feel inclined rather to attribute to them the same origin, and to 

 assume merely that the antennae had shifted further forward. This 

 conjecture is supported by a comparison of the antennae of the 

 Insecta with those of Peripatus. The former also as rudiments 

 show not only in form, which would there be less remarkable, but 

 also in position the closest agreement with the (primary) trunk- 

 appendages, indeed, to begin with they even lie post-orally (Fig. 147). 

 We might conclude from this that the antennae of Peripatus and 

 those of the Insecta were homologous structures, but that they could 

 not be compared with the cephalic tentacles of the Annelida, in 

 other words, that they were originally appendages of the primary 

 trunk and not of the primary cephalic region. 



If we accept this view we shall have to assume that the primary cephalic 

 region has greatly degenerated, and that a primary trunk-segment (the first) has 

 to a certain extent taken its place. An indication as to how and why this 

 happened is to he looked for in the fusion of the other and undoubted trunk- 

 segments in the adult head. The utilisation of the anterior limbs as mouth- 

 parts was accompanied by their partial transformation into sensory organs 

 (palps of the Insecta), and the final preponderance of one pair as feelers. 

 Again the brain would in this case have to be reckoned as belonging to the first 

 (primary) trunk-segment, and could not be derived from the neural plate alone. 

 This view, however, presents no difficulty when we see how, in Peripatus, the 

 ganglia of the maxillary segment passes from a post-oral to a pre-oral position 

 and is absorbed into the brain (p. 193). In the Crustacea the ganglia of the 

 second antennae undergo corresponding changes. 



The entire filling up of the so-called cephalic segment in Peripatus by a 

 regular pair of primitive segments with unbroken epithelial walls agrees with 

 what is found in a trunk-segment, but not with the condition of the cephalic 

 region in the Annelida. 



If the primary cephalic segment which, in the Annelidan Trochqphore, carries 

 the cephalic tentacles, has really undergone degeneration, we might expect to find 

 traces of this fact. The two small prominences which appear in front of the 

 antennal rudiments, the significance of which is still obscure, might be regarded 

 as vestiges of this kind (Figs. 90 and 94, a;). We might conjecture that they 

 are possibly vestiges of the primary Annelidan tentacles. This interpretation of 

 them, which appears to us very plausible, also leads to a striking agreement 

 with the Crustacea. In homologising the cephalic appendages of the latter 

 (Vol. ii., p. 166), a similar view was adopted, the same significance being ascribed 

 to the frontal sensory organs as is now given to the small prominences (.v) in 

 front of the antennal rudiments in Peripatus. 



It cannot be regarded as altogether improbable that the adult Peripatus 

 should still retain vestiges of this organ, the agreement of which with the 

 frontal sensory organs still functioning in many Crustacean larvae would be stil 



