66 Arenicolidae 



material, having little coherence, generally soon fall to pieces. In 

 any case the castings are inconspicuous among their surroundings, 

 and therefore do not betray the presence of the worms, as in the case 

 of A. marina. 



Nervous System and Sense-Organs. 



The brain is situated in the prostomium. In the caudate species 

 it may be compared in form to the letter H, oi', more accurately, to 

 two slightly flattened pears, lying side l)y side, with their narrower 

 faces adjacent and fused along the middle third of their length. The 

 more massive anterior portions of the pears represent the anterior, 

 and the tapering portions the posterior brain-lobes, the latter of 

 which lie below the nuchal organ. In the ecaudate species the brain 

 is of simple form — practically non-lobate, almost band-like — and 

 merges at each end into the oesophageal connectives. The brain 

 gives off anteriorly and posteriorly bundles of nerves to the pro- 

 stomial epithelium and the nuchal organ respectively. 



The oesophageal connectives arise, in the caudate species, from 

 the anterior and middle regions of the brain. In all species the con- 

 nectives run obliquely backwards and ventrally (PI. XIII, Fig. 46), and 

 unite with the ventral nerve-cord some distance in front of the first 

 chaetiferous annulus — that is, in the achaetous body-segment which 

 precedes the first chaetiferous segment. The course of the con- 

 nectives and the ventral nerve-cord is indicated externally, in some 

 species, by the metastomial and ventral grooves (Fig. 53, p. 118). 

 The oesophageal connectives give off nerves to those annuli through 

 which they pass, and to the statocysts. 



The ventral nerve-cord does not exhibit, in dissections, any 

 obvious signs of segmentation, its nerve-cells are not aggregated into 

 definite ganglia, but are distributed along the lateral and ventral 

 portions of the whole length of the cord. In all species, except 

 A. pusilla, there are, at segmental intervals, among the ordinary 

 nerve-cells, much larger "giant cells," which are connected with 

 remarkable " giant fibres." The ventral nerve-cord gives off a pair 

 of nerves to each inter-annular groove, and one or more pairs to each 

 chaetiferous annulus and its parapodia. 



The organs of special sense are the statocysts, the nuchal organ 

 (see p. 38), and the eyes. Other sensory structures are the pro- 

 stomium, the papillae of the "proboscis," scattered sense-cells in 

 the epidermis, the notopodial chaetae (around the bases of which 



