6oo THE ANIMAL KINGDOM. 



traversed by the duct of a paired poison gland (Langerhans). Glands may 

 be imbedded in the walls of the pharynx or appended to it externally. 

 The archenteric region is divisible into an oesophagus and a stomach- 

 intestine. The length and structure of the oesophagus vary much. In the 

 terrestrial Oligochaeta it is sometimes provided with a muscular gizzard, a 

 crop, and lime-secreting glands of Morren (p. 202). The glandular stomach 

 so-called of Syllidae immediately following the oesophagus is really a 

 muscular gizzard with radial columns of striated muscles in its walls 

 (Haswell). In some Syllidians and in Hesionidae it is furnished with a 

 pair of outgrowths which contain air, probably secreted from the blood 

 and derived in the first instance by the blood from swallowed air 1 . The 

 stomach-intestine is larger in calibre than the oesophagus, and is usually 

 more or less constricted by the attachment of the septa or of muscular 

 bands representing them. It is also usually pigmented. The pigment is 

 contained in the enlarged peritoneal cells (* chloragogen ' cells) which form 

 its outer coat, and very rarely in the lining epithelium. A pair of glands 

 may open into its commencement (e. g. Arenicola\ or a variable number 

 into its dorsal aspect (some terrestrial Oligochaeta, p. 202) ; and in Aphrodite 

 each of its segments has appended a right and left glandular caecum 

 branched at its distal extremity. The Capitellidae possess a ' siphon ' or 

 accessory intestine ventral to the main intestine and opening into it 

 anteriorly and posteriorly. A median longitudinal invagination of all its 

 coats forms the typhlosole of some terrestrial Oligochaeta (p. 203). The 

 coats of the oesophagus and intestine are an outer peritoneal cellular coat ; 

 an outer longitudinal and inner circular muscle-layer, sometimes, but very 

 rarely, reversed in position ; and the epithelium. The latter is usually 

 ciliated and often contains glandular cells. In the intestine its cells may 

 grow in size, be thrown off, and replaced by fresh cells as in some 

 Oligochaeta (p. 203) 2 . There is a short rectum, probably always derived 

 from a proctodaeum. The anus is usually terminal, sometimes dorsal. 

 The alimentary canal is as a rule straight, and rarely disposed in coils, 

 e. g. in Pectinaria and Siphonostomum among Tubicola. It is nearly 

 always supported by a dorsal mesentery which incloses the dorsal blood- 

 vessel. The ventral mesentery sometimes disappears completely, some- 

 times persists so far as to support the supra-nervian vessel. 



The vascular or so-called pseud-haemal system is absent in some 

 Polychaeta, i. e. Glyceridae, Capitellidae^ Polycirrus, Tomopteris, which may 



1 The two outgrowths of the oesophagus in Nereidae are possibly also air-bladders. The 

 intestine of Phyllodoce lamelligera has been found to contain air. The four families, Hesionidae, 

 Syllidae, Nereidae, Phyllododdae possess no branchiae. See Eisig, Mitth. Zool. Stat. Naples, ii. 

 1881, pp. 293-4. 



2 The epithelial cells of the hinder part of the intestine in some Syllidians contain refractile 

 spherules in large numbers. Claparede, supposing them to be of excretory character, termed this 

 portion of the intestine ' region urinaire.' 



