58 Arenicolidae 



and ecaudata the number varies in different specimens, the maximum 

 number seen in the former species is thirty pairs, and in the latter 

 forty-seven pairs. 



In all the species the first gill is almost invariably small, and, in 

 a considerable percentage of examples, is reduced to minute pro- 

 portions or is absent. The gills extend generally to the posterior 

 end in adult ecaudate examples : but from one to nine segments in 

 A. ecaudata, and from one to four in A. branchialis, may be gill-less. 



In the caudate species of Arenicola there are two different types 

 of gills, the pinnate and the fruticose (or bushy). In most specimens 

 the gills are readily referred to one or other of these types, but, 

 occasionally, it is difficult to state to which of the two forms the 

 gill belongs, that is, the two types merge into each other. In the 

 pinnate type the gill-axes are elongate, and along the sides of each 

 axis there are numerous ten to twenty opposite or alternating 

 branches placed at regular intervals, producing a pinnate appearance. 

 The subsequent division of the lateral branches is either dichotomous, 

 or, especially in the case of the larger ones, pinnate ; the ultimate 

 branches form the finger-like gill-filaments. The fruticose gill has 

 shorter axes, each of which bears few (three to six) lateral branches 

 on each side ; these branches are closely set or irregularly placed, 

 and they do not subdivide in a pinnate manner, but dichotomously, 

 or in such a manner that the ultimate branches, that is, the gill- 

 filaments, form a cluster. The collective effect of the massing 

 together of, say, eight or ten axes, similar to that described, is to 

 give the gill the appearance of a dense bush. 



The gills of A. cristata are invariably of the pinnate type. Those 

 of a specimen 175 mm. long are selected (PI. XIII, Figs. 41, 42) as 

 an average example of their size and condition in this species. The 

 first and smallest gill consists of eight axes, of which that shown in 

 Fig. 41 is the most dorsal and largest. It is 2 -8 mm. long and bears 

 nine branches on each side. Owing to the shortness of these branches 

 their mode of subdivision seems to be somewhat irregular ; some of 

 them dichotomise, while others subdivide almost at once into a 

 cluster of gill -filaments. The largest gills of the same specimen are 

 composed of about fourteen or fifteen axes. The dorsal axis, 7 5 mm. 

 long, of such a gill is shown in Fig. 42. The lower and middle 

 branches are subdivided freely, often in a pinnate, sometimes in a 

 dichotomous manner, and their terminal filaments lie, for the most 

 part, in one plane. In the figure these branches are represented as 

 seen in full view, but they actually lie in a plane almost at right 



