ON THE BRITISH ANNELIDA. 205 



are however soft, tenacious, flexible, and muddy. Slimy mucus, furnished 

 by the integumentary glands of the body, is the mortar or cement, fine sand- 

 molecules are the ' stones ' or solid material of the architecture. In the 

 SabellcB, the lime of which the tubes are built is held in solution in the 

 mucus provided by the cutaneous glands. It is adjusted in the fluid form, 

 and moulded by appropriate tools into the required shape : it then solidifies ; 

 solidifies too under water, like the "Aberthaio lime ! " The tube of the Sabellce 

 fits closely round the body of the worm ; it is slightly elastic, and the interior 

 is smooth. 



It is a fact of singular mechanical interest, that the thoracic feel, which vary 

 in number in different species, are disposed in a manner which is the exact 

 reverse of that in which the abdominal are arranged ; the latter being distri- 

 buted over the posterior iths or fths of the body. This remarkable pro- 

 vision confers manifestly on the inhabitant of the tube, which is frequently 

 unattached except at its inferior end, the power of rolling on its own axis, of 

 turning round, in order that it may sweep with its branchiae in search of 

 food and fresh portions of water, the whole circumference of the circle. This 

 disposition of the thoracic feet on the dorsal aspect of the body, while the 

 abdominal are placed on the ventral, with a simplicity of mechanism per- 

 fectly wonderful, arms the worm with the means oi fixing the tube whilst it 

 executes its complex movements. If all the feet were disposed on the same 

 side of the body, this important object, it must be at once obvious to the 

 mechanician, never could be accomplished. The W'Orm would be a palsied 

 prisoner in its self-constructed cell. 



The brushes of setae in the thoracic feet point dorsally, and the row of 

 hooks extend from their bases in the direction of the dorsal median line. The 

 brushes of the abdominal feet point ventrally, and the row of hooks extend 

 from their roots transversely in the direction of the ventral median line. It 

 is evident then that these classes of feet must act in opposite directions. The 

 hooks slightly vary in form in different species, but the setae in all the species 

 are constructed on one plan (fig. 23, a). In Sabella a sang vert (M. Edwards), 

 the hooks (fig. 23, B) are formed by the turning of a finely sharp beak very 

 much upon itself, the attached»end or root expanding into a broad base. 

 From this latter part a very curious claw-like process, surmounting a straight 

 shaft, proceeds (fig. 23, B), the use of which must consist in tightening the 

 tube after the hook has been fixed, and that to render the hold of the latter 

 more secure. The setce present a leaf-like form, and margins strongly toothed 

 (fig. 23, a). Composed of an unyielding horny material, they are admirably 

 fitted for pushing ; the feet in these, as in other worms, act in obedience 

 to the principle of the composition of forces. The forces meeting and uniting 

 in the ideal axes of the body, or rather in the centre of the body of the 

 animal, produce a resultant expressed by its raoWon forward. 



In Sabella vesiculosa (Montagu), the hooks are formed somewhat differ- 

 ently. The broad bases observed in the former instance, are replaced by a 

 tapering curved end to which the muscle is attached (fig. 23, B). The feet 

 are similarly distinguished into thoracic and abdominal : the setae are con- 

 structed upon the same precise type. The same disposition of the feet pre- 

 vails in Sabella unispira (M.-Edwards) ; here, however, the bases of the 

 hooks are broad, and the claw-like process described in S. a sang vert reap- 

 pears. The setae preserve the type of construction characteristic of the genus. 



The feet in Terebellce are composed only of hooks and setae. The soft 

 appendages are transferred to the head, where, under the form of tentacles, 

 they assume an extreme degree of development. A full statement of the 

 history of these remarkable organs has already been given. 



