Gills of Arenicola 



61 



cristata, both of which have eleven pairs of gills, are the extreme 

 terms in the gill-series : in the former the axes are reduced and the 

 branches clustered, in the latter the axes are elongate, and have 

 numerous pinnately arranged branches. 



The gills of the two ecaudate species, A. ecaudata and branchialis, 

 which are identical in form, differ markedly from those of the 

 caudate species. One of the smaller gills of a Neapolitan example 

 of A. 'branchialis (76 mm. long) is shown in Fig. 35. Arising from a 

 short common trunk are three stems about 1 mm. long, each bearing 

 three branches, which are given off on one side only. The sub- 

 divisions of these branches are similarly restricted to one side, and 

 the division is invariably dichotomous. The larger gills of the same 



Fig. 35. A. branchialis, entire gill. 



Fig. 36. A. ecaudata. 

 Dorsal gill - stem. 

 The gill had four 

 other stems similar 

 to this, all united 

 at their bases. 



specimen have four or five main stems with more numerous 

 branches. A careful examination of many gills gives one the 

 impression that the stem is not a true axis but a sympodium. A 

 dorsal gill-stem of a British example of A. ecaudata (190 mm. long), 

 represented in Fig. 36, exhibits the same mode of branching. 



COELOM AND COELOMIC SEPTA. 



The coelom is spacious and continuous from one end of the 

 animal to the other. In front it is traversed by three transverse 

 septa placed at the anterior boundary of the first, third and fourth 

 chaetiferous segments respectively. These septa are present in all 

 species of the genus. 



The first septum is the strongest, for a portion of the pharyngeal 



