RELATION OP ARTHROPOD HEAD TO ANNELID PROSTOMIUM. 251 



kind is offered to us in the case of the loss of the limbs in 

 snakes. The argument which might be urged — that the an- 

 cestors of the Ophidia were legless, since no obvious vestiges 

 of limbs are seen in by far the greater number of snakes at 

 any time in their development — is entirely disproved by the 

 few instances, such as the Python and Tortrix, in which such 

 vestigial hind limbs are known to occur. 



Segments may be suppressed, either temporarily in the 

 young, as in the zocEa larva ; or, on the contrary, in the adult, — 

 as, for instance, the first abdominal segment in Arachnida. On 

 the other hand, neglecting the special cases of reproduction 

 by fission, new segments are never intercalated between old 

 ones, except in the normal process of growth at the tail end 

 of the animal. This brings us indeed to one of the most 

 important characters of the segmentation of Annelids and 

 Arthropods, namely, that new segments, during the develop- 

 ment of an individual, are invariably added between the last 

 segment or telson, and the one immediately in front of it. 

 All apparent exceptions to this rule, often called the law of 

 Milne-Edwards, seem to be due to retardation in development, 

 as in the case of the zooea alreadv mentioned. 



At the risk of wearying the reader, it has been necessary to 

 indulge in these commonplace and well-known remarks for the 

 sake of clearing the ground. We may now return to the 

 discussion of the morphology of the peristomium in Annelids. 



Careful modern researches (Vejdovsky, Wilson, &c.) have 

 shown that in Oligochsetes the peristomium exhibits the essen- 

 tial characters of a true segment. It develops as a region 

 surrounding the mouth, in which are formed a pair of meso- 

 blastic somites which become hollowed out to form the 

 coelom ; a ganglionic thickening is produced ventrally, which 

 soon fuses with that of the succeeding segment ; a uephridium 

 (head kidney) is developed. In the Polychsetes, on the other 

 hand, where the head in the larva is so often enlarged to a 

 disc-like shape, it is generally more difficult to trace the origin 

 of the coelom in the peristomium, as indeed also in the seg- 

 ments behind it. In some cases, at all events, it has been 



