94 ADAM SEDGWICK. 



(2) In Medusae there are a number of pores leading from 

 the circular canal to the exterior, placed on the oral side of the 

 insertion of a radial tentacle, i. e. opposite a radial canal ; in 

 Actinozoa there are a number of openings in the body wall, 

 putting the pouches in communication with the exterior (for 

 function and possible origin of these pores vide below, p. 96). 



(3) In Medusae the circular striated muscles of the sub- 

 umbrella are interrupted by the radial canals (Her twig) and 

 so broken up into a number of segments. 



(4) In Medusae there are sense organs which may be in 

 connection with special nervous aggregations (Acraspedaj at 

 the periphery of each radius. 



In segmented animals — 



(1) When segmented appendages are present (Arthropoda, 

 Polychaeta) they are simply processes of the body wall con- 

 taining prolongations of the body cavity (Peripatus, embryonic 

 Arthropoda). 



(2) The nephridia are essentially pores leading from the 

 body cavity to the exterior on the neural side at the base of the 

 appendage. 



(3) The muscular system is sometimes broken up into bands 

 corresponding to the segments. 



(4) The nervous system sometimes presents swellings, one 

 for each somite. 



I further venture to suggest that the greater number of the 

 Triploblastica have arisen from diploblastic animals with a 

 pouched gut ; that in some of these, in consequence of the form 

 taken by the body (elongation) and the consequent necessity 

 for jointing and the persistence and greater development of 

 the paired appendages, the body has become moulded, so to 

 speak on this primitive gut pouching, which has therefore left 

 its trace in the " segmentation " ; that in unsegmented Triplo- 

 blastica, in consequence of the action of causes of an opposite 

 nature to those just mentioned, the pouches, after becoming 

 separated from the gut, have become completely con- 

 tinuous with one another and left no traces. As a known 

 instance of the latter process I may mention Echiurus 



