coe: nemerteans of west and northwest coasts. 75 



A free-swimming blastiila forms in a few hours, and from tliis the 

 gastriila develops. The invagination apparently involves not only 

 the entoderm but also the neighboring ectoderm cells, so that the 

 primitive enteron which is thus foimed consists of two chambers — 

 the broad esophagus, composed of ectoderinic cells, leading into the 

 second, smaller chamber, the blind mid gut, or stomach, which is 

 lined with tlie entoderm (Texts-figs. 29, 30). 



In the course of two days the gastrula develops into the pilidium 

 (Text-fig. 30), for the structure of which the reader must be referred 

 to previous papers (Coe, '99o ; Btirger, '97-: 03). The structure of 

 the pilidium of other forms has been studied by Metsclinikoff ('69), 

 Salensky ('84), Biirger ('94), Coe ('99''), C.B.Wilson (:00), and 

 others. 



The complicated metamorphosis by which the body of the young 

 worm develops within the pilidium has been described by Krohn 

 ('58), Leuckart and Pagenstecher ('58), Metschnikoff ('69), Biitschli 

 ('73), Salensky ('86), and Biirger ('94). An excellent account of 

 these processes so far as they are at present known is given by 

 B:irger in Broan's Thierreich (vol. 4, supplement, p. 338-366). 



Geographical Distribution. 



The total number of species recorded in this report from the 

 Pacific coast of Noilh America is much larger than was to have 

 been expected, for there are now more species of nemerteans de- 

 scribed from this coast than from any other region of equal size on 

 the globe, although as yet comparatively few attempts have been 

 made at their collection. And, as has been noted elsewhere, we find 

 in these localities not only so large a number of species, but also a 

 surprising abundance of individuals of the forms represented. And 

 certainly I know of no region where so large a proportion of the 

 invertebrate animals found in a miscellaneous collection belongs to 

 the nemerteans as on the noithwest coast of North America. 



The study of these collections has had an increased interest 

 because of its bearing on the geographical distribution of cei-tain of 

 the species represented. 



Of the 86 species at present known from the Pacific coast of 

 North America and the coasts of Alaska bordering the Bering Sea 

 and Arctic Ocean, only 10 have as yet been recorded from other 



