ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 421 



discussed when the modifications of structure and develop- 

 ment of each family are known as a whole. With reference 

 to the Scalibregmidte, for example, many of the structural 

 features are only known through the work of Rathke (1848) 

 or of Danielssen (1859). Even more recent writers, such as 

 Wiren (1887), have been content, when dealing with the 

 vascular system or the subdivision of the body-cavity, to 

 record the results of the examination of one or two genera 

 without even determining the limits of specific differences, 

 which at any rate in the Arenicolidae extend from obvious 

 external marks to the mode of origin of the gonads, or the 

 grade of development of certain ganglion-cells in the nei've- 

 cord. 



But there is yet another class of evidence quite as impor- 

 tant for phylogenetic deduction as the modifications of organs 

 or the changes in larval and post-larval history. We refer 

 to the power of response to stimulation, or, in other words, the 

 physiology of the group. It is generally assumed that the 

 members of the sub-order of Polych^eta to which Benham 

 has given the term Phanerocephala Scoleciformia,^like 

 those of some other sub-orders, have acquired their distinc- 

 tive grade of organisation, in part at least, in adaptation to 

 their burrowing or sedentary mode of life. For example, the 

 reduction of the brain, the small size of the prostomium, the 

 absence of an armed pharynx, the modifications of the setse, 

 are explained by reference to the loss of free life, lack of active 

 competition, and to the necessit}^ of acquiring a structure 

 conforming to a less complex mode of life than is enjoved by 

 the Nereidiformia. A test which is quite applicable to both 

 groups, since they are in a large sense living under similar 

 conditions, would be the sense of response to variations of 

 light-intensity at different phases of growth, and to other 

 well-known forms of stimulation. At present we have abso- 

 lutely no data. The noteworthy presence in the Capitellidte 

 of one form, Dasybranchus, which possesses a brain more 



' Includin;^ Oplieliidse, MaKlaiiidaj, Arenlcolidre, Scalibrei^midse, Chlorae- 

 niidse, arid Steriiaspidfe. 



