ANELLATA. 135 



its walls are thickened by a stratum of minute secerning cells (d), 

 which prepare a greenish -yellow fluid. 



The alimentary canal commences in many of the Nereids by a pro- 

 boscis formed by a loose and muscular cylinder, which can be in- 

 verted and protruded like the finger of a glove ; its extremity in some 

 species is encircled by smal papillae; in some it supports a rasp -like 

 horny plate ; in others it is armed by one or more pairs of lateral 

 horny dentated jaws. 



In most of the Nereids there is no distinction between stomach and 

 intestine ; in some species the canal is provided with lateral pouches. 

 In the Aphrodita aculeata the part analogous to the projectile pro- 

 boscis of the Nereids is converted into a kind of gizzard, by the 

 thickening of the muscular coat. The alimentary canal, continued 

 from its posterior extremity, bends forward at first for half the length 

 of the gizzard, a disposition which indicates the occasional protrusion 

 of this part. The canal then bends backwards, and is continued 

 straight to the anus. Through the whole length of the intestine, 

 caecal processes are sent off on each side, to the number of about 

 twenty pairs. They commence of a slender diameter, but gradually 

 enlarge, send off" many short branches, which subdivide, and terminate 

 in fusiform csecal pouches. These productions of the intestinal canal 

 seem obviously analogous to the gastric caeca of the leech ; but they 

 are more isolated from the common canal^ and more distinct in their 

 functions. It is thought that the chyme passes into them, and that 

 the chyle is separated from it by a secretion of the terminal caeca 

 analogous to bile. Hunter has placed this preparation * at the 

 commencement of his series of hepatic organs, as one of the early 

 forms of that system. 



In the majority of the Anellides the blood is red ; in some of a 

 brilliant red, as in the Arenicola, Nereides, Glycera, Nephtys. In the 

 Aphrodita acideata and in the Polynoe, the blood is of a pale yellow 

 colour ; in a species of Sahella it is olive green ; so that, as Milne 

 Edwards well observes, the colour of the blood is far from being a 

 character of such physiological importance as to justify the location 

 of the Anellides at the head of the articulate sub-kingdom. In all 

 the species the blood is characterised by granulated circular cor- 

 puscles, of very variable dimensions, in the same animal. It circulates 

 in a closed system of arteries and veins, the modifications of which 

 are considerable when examined in the different genera of the 

 class. 



A large vessel which rests on the digestive tube, is the seat of 



* No. 782. 

 K 4 



