246 ANNELIDA. 



rous vascular tufts, a pair of which are appended to the outer surface of 

 every ring of the body, or, in some cases, only to a few of them. The 

 organs of locomotion, which are attached to each segment, assume various 

 forms, hut are generally composed of short moveable spines, or packets 

 of retractile bristles, usually destined to perform the office of oars. In 

 the annexed figure (fig. 118,1), which represents Laodicea antennata, the 

 general form of these animals is well seen, as is the most usual arrange- 

 ment of the branchial tufts and locomotive setae. In fig. 118, 2, showing 

 an imaginary transverse section of one of the segments, the relative 

 positions of the oars (c, d, e) and of the branchial appendages (b) are 

 likewise indicated. 



(643.) But the organs of respiration in the Dorsibranchiate Anne- 

 lidans are not always arborescent ; on the contrary, they are not unfre- 

 quently spread out into thin membranous lamellae, or resemble fleshy 

 crests or vascular tubercles : still, whatever their form, their office is the 

 same ; and the vessels spread over them presenting an extensive surface 

 with which the water is brought in contact, the blood is oxygenated 

 as it passes through them. 



(644.) The second class of organs to be enumerated as entering into 

 the composition of the lateral appendages are soft, fleshy, and subarti- 

 culated processes called cirri (fig. 118, 2, c, e) ; these are generally two 

 in number, and belong one to the ventral and the other to the dorsal 

 oar : their precise office is not well understood ; but as in some of the 

 segments, especially in the neighbourhood of the head, they assume a 

 tentacular form, they have, with much probability, been regarded as 

 instruments of touch. 



(645.) The setce (fig. 118, 2, d) are, perhaps, the most efficient agents 

 in progression. These are long and stiff hairs disposed in bundles and 

 implanted into strong muscular sheaths. Each packet of setae can be 

 retracted within the body to a certain extent, and again protruded by 

 the action of the tubular supports from which they arise ; and being- 

 capable of independent action, these organs must be looked upon as so 

 many powerful fins, well calculated to propel the creature through the 

 element it inhabits. 



(646.) The structure of the mouth in the Dorsibranchiate Annelidans 

 is very peculiar. The first portion of the alimentary canal, or stomach, 

 as it is most erroneously called by some writers, is muscular ; and cer- 

 tainly, when seen in a dead Annelid, it might easily be taken for a 

 digestive cavity. Nevertheless, during life, this part of the alimentary 

 apparatus is destined to a widely-different office ; for it is so constructed 

 that, at the will of the animal, it can be completely everted, turned 

 inside out, and, when thus protruded externally, it forms a very singular 

 proboscis, used in seizing food, and frequently armed with powerful 

 teeth of singular construction. The following figure (119, A), repre- 

 senting the head of one of these worms (Goniada d chevrons, Milne- 



