ON THE BRITISH ANNELIDA. 219 



cessive portions of the body. In all Annelids the swelling of certain por- 

 tions of body in progression is accomplished by aid of the fluids of the inte- 

 rior. This is driven to a given point of the containing cavity, and then 

 momentarily imprisoned there by the contraction of the circular integument- 

 ary muscles in front of it and behind it. Hereat, for a moment, the body 

 bulges. The muscles of the integument are then excited to action, and the 

 fluid is forcibly compressed forwards or backwards, according to the direc- 

 tion of the muscular agency. This is a summary exposition of the mecha- 

 nical uses of the chyl-aqueous fluids of the peritoneal cavity, of which the 

 vital and physiological meaning was formerly studied in extenso. 



Nearly all Annelids are struck with paralysis when this fluid is made to 

 escape from its cavity by a puncture through the external walls. The power 

 of voluntary motion is suspended. The body of the worm becomes passive 

 and flaccid. The peritoneal fluid is really the fulcrum on which all muscular 

 action is based. Without it the worm cannot direct the contraction of its 

 muscles with efficiency and precision. But its mechanical uses are not ex- 

 clusively limited to the aid aff"orded in progression. It prevents mutual and 

 injurious pressure amid the internal organs, without which the course of the 

 blood in its proper vessels is arrested. In the leech-tribe it is the fluid which 

 is contained within the stomach that accomplishes this important object. 

 This singular anatomical peculiarity is also observed in the Liniadce. Nothing 

 in the history of the Annelids can be conceived more wonderful than the 

 mechanically perfect and facile manner in which Linus longissimus, a worm 

 of many yards in length, performs the feat of locomotion, and that too over 

 craggy and rugged rocks. Without the fluids of the body, its motor appa- 

 ratus would be incapable of eff'ort. 



Alimentary system In the majority of Annelids the alimentary system 



constitutes a cylindrical tube, which bears a general resemblance of outline 

 to the integumentary, this latter forming with respect to the former an ex- 

 terior concentric or embracing cylinder. As formerly explained, these two 

 cylinders are in no instance in agglutinated contact; a space intervenes, 

 varying in capacity in different species, to designate which the term ' peri- 

 toneal' or 'splanchnic' may be used with perfect anatomical propiiety. This 

 space is occupied by a vital or organized fluid, charged with corpuscles, 

 which discover under the microscope characters distinctive of species. In- 

 dependently of its physiological uses, this fluid enacts mechanical functions 

 indispensable to the well-being of the animal. On it, as upon a pivot, the 

 vermicular motions of the intestinal cylinder are performed. In locomotion 

 the shortening and lengthening of the body in many species are quite extraor- 

 dinary. The alimentary canal participates in this longitudinal motion. In 

 the small species, as the JVa'ides, having transparent integuments, the longi- 

 tudinal play of the intestine, running as it were backwards and forwards 

 through the integumeiitary cylinder with great rapidity and precision, may 

 be readily observed. This motion, more or less appreciable, occurs even in 

 those species least gifted with the power of elongation. The septa of the 

 segments approximate under the action of the longitudinal muscles, and the 

 included portion of the intestine is shortened. This process, multiplied by 

 the number of segments in the body, will give a considerable resultant of 

 aggregate longitudinal motion. 



Although as a whole forming a cylinder, in no instance does the ali- 

 mentary canal present the figure of a smooth-walled tube. The parietes are 

 invariably sacculated, and often superficially multiplied in the most elaborate 

 manner. In the lumbriciform species each segment of the body has its own 

 independent stomach. Those of contiguous segments communicate through 



