164 COMPARATIVE ANATOMY. 



lamellEe, covered with epithelium. There are clefts between the 

 branchial arches and the several lamellae which go to form them ; 

 these lead, on each side, to a series of spiracles, by which they open 

 on to the surface of the body. A network of vessels is spread over 

 the branchial framework. The water taken in by the mouth streams 

 through the superior respiratory semi-canal into this branchial 

 apparatus, and passes again to the exterior by the rows of spiracles. 

 The hind-gut of many Annelids may be seen to take in water, 

 and this may be connected with a respiratory function of this division 

 of the intestine. No special organs of respiration have been ob- 

 served to be developed in this portion. It is not yet certain 

 whether the structures which are found in the hind-gut of Neomenia 

 are really branchiae. More exact anatomical knowledge of the pro- 

 tractile branchiae of CliEetoderma is necessary before we can say 

 what is their morphological signification. 



Accessory Organs of the Alimentary Canal. 



§ 134. 



The enteric canal of the Vermes has glandular organs of various 

 kinds connected with it, which are to be regarded as differentia- 

 tions of the enteric wall, that is to say of the endoderm. Single 

 cells, or groups of cells, acquire a different character from their 

 neighbours, and so give rise to special organs, which are of different 

 degrees of individuality, according as their position is on the 

 enteric wall, or without it ; in the latter case they are connected 

 with the lumen of the intestine by ducts. They may further be 

 more exactly classified, according to their relation to the separate 

 divisions of the enteron. 



In the fore-gut, close behind the muscular pharynx, small groups 

 of unicellular glands open in the rhabdocoelous Turbellaria. In the 

 Trematoda similar groups of cells are placed at the anterior end of 

 the body, opening near the mouth, and are regarded as pharyngeal 

 glands. Glandular organs have been observed in the so-called 

 pharynx of the Nematodes, and distinct glandular cells in the oral 

 region. 



Among the Annulata the histology of the Hirudinea, especially, 

 has been carefully investigated ; in them, a large number of uni- 

 cellular glands open into the proboscis, when there is one, or into 

 the jaws, when they are present. In the Annelides a pair of lobate 

 glandular tubes are found in the last division of the fore-gut, just 

 behind its muscular portion in the Nereids and others provided with 

 pharyngeal jaws (cf. Fig. 55, gl) ; they are modifications of the 

 simpler tubes of the Syllidae. The Rotatoria are provided with 

 glandular appendages at the same spot. It is usual to call these 

 glands, which have however various kinds of functional relations, 

 '' salivary glands." 



