140 COMPARATIVE ANATOMY CHAP. 



entirely wanting). In the 3 families mentioned it consists of a 

 simple oesophageal tube between mouth and intestine (Pharynx 

 simplex, Fig. 102, A}. This simple oesophagus becomes at first 

 complicated by the massing of definitely arranged muscles around it. 

 The muscular wall of the oesophagus, then, almost always projects in 

 various ways more or less far into the lumen of the oesophagus, so that 

 we can now distinguish in the pharyngeal apparatus 2 chief parts : (1) 

 the cesophageal or pharyngeal pouch, and (2) the muscular cesophageal 

 bulb or pharynx which projects into it. When the oesophageal pouch 

 is not spacious, and the pharynx with its free inner surface projects 

 only a short way into it, the latter is generally round or barrel- 

 shaped, and is sharply marked off from the body parenchyma which 

 surrounds it by a layer of muscle (Pharynx bulbosus, Fig. 102, B}. 

 In this form we meet it in nearly all Rhabdoccela, in the Plagiostomidce 

 among the Alloioccela, and further in all Trematoda. 



In very many Turbellaria the pharynx projects as a circular fold 

 into the mostly spacious pharyngeal sheath or pouch, and takes, like 

 the latter, very various shapes ; it is in this case never sharply severed 

 from the surrounding parenchyma by a muscle layer (Pharynx 

 plieatus). In all Polydada, with the exception of Euryleptidce and 

 Prosthiostomidce, the pharyngeal pouch is very spacious, and often has 

 secondary pouches, which again are occasionally branched ; and the 

 pharynx is a flat and broad band which hangs as a circular fold from 

 the sides of the pouch (Fig. 102, C). Such a pharynx can be extended 

 through the oral aperture, and, spreading out, envelop its prey on all 

 sides, as in a sheet. In the En.ryleptidce and Prosthiostomidce among 

 the Polydada, and in all the Triclada, and in the Monotidce among the 

 Alloioccela, this circular fold becomes a more or less extended cylin- 

 drical muscular tube, which projects freely into the equally cylindrical 

 pharyngeal pouch from its base. By contraction of the circular 

 musculature this tube elongates and passes out through the oral 

 aperture (Fig. 102, D}. 



The relation of the pharynx plieatus to the pharyngeal sac is 

 similar to that of the variously-shaped oral or gastric peduncle of the 

 Acraspede Medusa to the subumbrellar cavity into which it projects. 



The musculature of the Platode pharynx consists of one or more longitudinal 

 and circular muscular layers, and of muscle fibres arranged radially round the axis of 

 the pharynx. 



All over the free surface of the pharynx, and chiefly at its free end, unicellular 

 glands (salivary glands) open. These glands lie either in the pharynx itself, as in 

 the pharynx bulbosus, which is sharply marked off from the parenchyma, or, as in the 

 pharynx plieatus, scattered about in the parenchyma round the place of insertion of 

 the pharynx. In the latter case they send only their long and thin processes 

 (efferent ducts) into the pharynx. 



The following is the rule for the position of the pharynx and the pharyngeal sac. 

 When the mouth lies in the middle of the body the enteric aperture is found directly 

 above it. The axis of the pharynx and its pouch then stand perpendicularly to the 

 ventral surface. If the mouth lies to the front the enteric aperture lies behind it ; 



