ENTEEIC BEANCHLE OF VEEMES. 163 



jaws. Tliere is sometimes only one pair of these jaws (Fig. 55, m), 

 and sometimes tliere are several, which differ from one another in 

 particular characters, and form a complicated apparatus (Fig. 75). 

 This portion is very gi^eatly developed in the Aphroditeida?. It 

 can be protruded, as in many other voracious Annelids (Phyllodoce, 

 Glycera, etc.), so that the anterior portion is everted and forms the 

 outer surface of the " proboscis." This protrusible portion is some- 

 times very long. 



The whole arrangement undergoes atrophy in the Tubicolae, to 

 which Ai'enicola is an intermediate step. The third division of the 

 fore-gut is feebly developed in the Scoleina, and more so in the 

 Cheetopoda, where it is often seen to be provided with a pair of 

 ca3ca (Syllis, Arenicola). 



The mid-gut forms the largest, and also the most uniform, portion 

 of the whole enteric tube. It generally has a perfectly straight 

 course, and is seldom disposed in coils or loops. It is not only 

 attached by the muscular lamellte from the body-wall, or by the 

 separate fibres from the edges of the metameres, but is also divided 

 into separate, and often diverticulate, portions, which correspond to 

 the metameres. In the family of the Aphroditeidje, as in the 

 Hirudinea, diverticula of this kind are developed into larger appen- 

 dages, which may exhibit numerous branches (Fig. 74, c). 



The hind-gut is generally a short portion, and is of some size in 

 the Tubicolae and in Arenicola only ; it seldom has a median en- 

 largement, and generally extends to the anal ojDening from the 

 mid-gut, without being sharply marked off from it. 



Myzostoma agrees with the Annelides in the character of its 

 intestinal tube. The fore-gut is represented by a long protractile 

 proboscis, which leads into a widened mid-gut, from which a narrower 

 hind-gut leads to the anus. Branched ceeca are distributed through 

 the body from either side of the mid-gut. 



Enteric Branchiae. 



§ 133. 



The development of the respiratory function of the enteric tube 

 leads to special arrangements in it, which are greatly developed in 

 Balanoglossus. The anterior portion of its enteric tube is separated 

 by two lateral processes into two semi-canals, which lie one above the 

 other, and communicate freely by means of the longitudinal opening 

 which lies between the two laterally outstanding'processes. The semi- 

 canal, which I regard as the lower one, leads directly to the commence- 

 ment of that portion of the enteron which functions exclusively as a 

 nutritive canal. The investment of cilia drives the particles of food 

 into it ; it has a nutritive function. The semi-canal, however, which 

 is placed dorsally, has a respiratory function. There is a branchial 

 support in its walls in the form of a delicate framework of chitinous 



M 2 



