ANNELIDA. 



169 



Fig. 68. 



Fig. 69. 



we may observe that these ccecums are replaced 

 by blind canals, either simple or ramified; 

 thus in the arenicola, or sand worm, we find 

 that there communicate with the second sto- 

 mach two ccecums terminated by a soft point, 

 with thick parietes of a yellow colour ; and in 

 the aphroditae the stomach opens on either side 

 into a score of membranous appendages, which 

 commence of very contracted diameter, but 

 afterwards insensibly become dilated and di- 

 vide into many branches : (see Jig. 70, a, the 

 retracted probos- 



Fig. 70. cis, b bj the ap- 



pendages.) This 

 type of structure 

 leads to that which 

 is manifested in 

 the planariae, and 

 also approximates 

 to what one sees 

 in the parasitic 

 arachnida. 



The intestine 

 which succeeds 

 the stomach is 

 generally narrow, 

 and in the majo- 

 rity of the anne- 

 lida extends in a 

 direct line to the 

 anus. In some 

 species, as the 

 amphitrites, it 

 presents a greater 

 or less number 

 of convolutions. 

 There does not 

 exist in these 

 animals a gland 

 which can be re- 

 garded as a liver, 



properly so called : the appendages which are 

 grouped around the stomach in the arenicola: 

 may, indeed, be biliary vessels analogous to 

 those of insects rather than true cceca ; but in 

 the earthworms and many other annelides the 

 bile would seem to be secreted by a peculiar 

 organ of a yellow colour and pulpy texture, 

 which surrounds like a sheath a great part of the 

 digestive canal. Lastly, in certain annelida, as, 

 for example, the thalassemae, there exists on 

 either side of the oesophagus a small organ, 

 which would seem to have a secretory office, 

 and may very probably be a salivary gland.* 



Circulation. The blood in almost all the 

 annelida differs from that of every other in- 

 vertebrate animal by its red colour; some- 

 times, however, this fluid has scarcely a tinge. 

 According to M. De Blainville the blood of 

 the aphroditae is yellow, and MM. Mayor and 

 Gosse, of Geneva, assert that the circulating 

 fluid of the genus clepsina, one of the hirudi- 

 nidae or leech-tribe, is even altogether white. 

 When the blood of an annelide is examined 

 with the microscope it is seen to contain circu- 

 lar globules, but of a much larger size, and in 

 far less number than in human blood : it coa- 

 gulates after rest like the blood of the higher 

 animals, but it appears to contain a very small 

 proportion of fibrine. 



The blood circulates, as we have already 

 stated, in peculiar vessels, which its red colour 

 renders easily distinguishable. 



The vascular system has been best studied 

 in the earthworm : above the alimentary canal 

 there runs along the entire length of the body 

 a contractile vessel (fig. 71, a,) which is con- 

 sequently dorsal, and in which 

 the blood passes generally 

 from behind forwards, some- 

 times in large waves, some- 

 times by small quantities pro- 

 pelled by the successive con- 

 tractions of the divisions which 

 this vessel forms through its 

 entire extent. A portion of the 

 circulating fluid then passes 

 into another vessel (c), which 

 originates at the anterior ex- 

 tremity of the one above- 

 mentioned, and which runs 

 backwards along the ventral 

 surface of the body below the 

 nervous column, from which 

 circumstance it has been cal- 

 led the sub-nerval vessel by 

 Duges. But the greater part 

 of the blood which is con- 

 tained in the dorsal vessel, in- 

 stead of following this chan- 

 nel, passes into seven or eight 

 pairs of large lateral branches 

 composed each of a series of 

 dilatations or rounded ve- 



* See Willis, ' De Anima Brutorum ;' Pallas, 

 * Miscellanea Zoologica ;' Cuvier, ' Anat. Comp.' 

 Treviranus, op. cit. Moquin Tandon, op. cit. ; 

 Duges, op. cit. j Home, < Lectures on Comp. 

 Anat.' 



Fig. 71 



