202 



COMPARATIVE ANATOMY 



CHAP. 



ment. The fore-gut is limited to the head segment, and forms a single 

 pharyngeal cavity Avith a weakly developed muscular wall. In all 

 other Oligochceta the fore-gut stretches through several segments, and 

 is divided into two parts by a transverse constriction, an anterior 

 part, the buccal cavity, and a posterior part, the pharyngeal cavity or 

 pouch. The dorsal wall of the pharyngeal cavity is nearly always con- 

 siderably thickened, and projects into the pharyngeal cavity in the form 

 of a variously shaped muscular pharynx. The pharynx is attached by 

 muscles to the body wall, and can be protruded in order to take in 

 food, the pharyngeal pouch being at the same time necessarily everted. 

 Various glands pharyngeal glands, salivary glands, and septal glands- 

 may enter the pharyngeal pouch. The part of the intestine which in 

 the Oligochseta follows the pharyngeal pouch and is generally termed 

 the oesophagus belongs, according to recent ontogenetic investigations, 

 to the (endodermal) mid-gut, and will thus be treated of later. 



Among the Polychceta we again come upon very various forms of 

 the fore-gut. In most of the tubicolous forms it is a short soft-skinned 

 division which fellows the mouth, and is called the oesophagus. In 

 Terebellida} the oesophagus carries a ventral muscular appendage, the 

 cesophageal sac. Most of the Polychceta, however, are characterised by 

 the possession of a pharyngeal apparatus, which, especially in the 

 Errantia, reaches a high degree of complication and can stretch through 



A 

 rt jtl> g 



^^^^^^^fT>', 



FIG. 132. Diagrammatic representation of the pharyngeal apparatus of a carnivorous 

 Annelid, g, Brain ; ph, pharynx ; l\ jaw ; m, mouth ; rt, retractors ; pt, protractors ; vt, anterior 

 soft-skinned portion of the pharyngeal apparatus ; p, its papilla?. A, Pharyngeal apparatus in a 

 withdrawn condition. B, In a protruded condition. In B, ct indicates retractors ; rt, anterior soft- 

 skinned portion of the pharyngeal apparatus. 



many segments. We may in a general way distinguish three modi- 

 fications of this pharyngeal apparatus. 



1. The pharyngeal apparatus consists of two portions. The anterior 

 portion, into which the mouth leads, is a soft-skinned tube, which 

 is often provided internally with papillae. The wall of the posterior 

 portion is thick, on account of the strong development of its muscular 

 layers, and represents the actual pharynx (generally called proboscis). 

 Its anterior end carries papillae projecting inwards or a conical process, 

 or besides these (Errantia} two hard chitinous jaws. This pharynx can 

 be so pushed forward that its anterior end which is thus armed pro- 

 jects freely outwards, and is then everywhere surrounded by the 



