230 INVERTEBRATE MORPHOLOGY. 



of longitudinal and circular fibres ; and in addition, between 

 these, a layer composed of fibres which cross one another 

 obliquely is usually present. A marked distinction from what 

 occurs in the Chsetopoda is found in the ccelom, which in the 

 Hirudinea is traversed by a parenchyma, recalling that of the 

 Platyhelrninths, so that the actual cavity is to a great extent 

 obliterated, and the dissepiments only to be distinguished with 

 difficulty. Those portions of the ccelom which persist (Fig. 

 107, Cc) are occupied by a red or colorless fluid containing 

 corpuscles and identical and continuous with that found in the 

 blood-vessels. The ccelom is in fact represented by a number 

 of blood-sinuses, which in some forms are lined by an epithe- 

 lium, while in others such a lining is wanting. On account of 

 the manner in which the blood-vessels anastomose with the 

 sinuses it is exceedingly difficult to distinguish which spaces 

 should be considered as belonging to the circulatory system 

 proper and which to the ccelorn if, indeed, the two are to be 

 considered fundamentally distinct. As a rule four main lon- 

 gitudinal vessels or sinuses are to be found viz., one dorsal 

 (Fig. 107, ds), which may be wanting (Nephelis, Fig. 107, C) and 

 which probably corresponds to the dorsal vessel of the Chre- 

 topoda ; one ventral (vs), sinus-like in character and frequent- 

 ly destitute of an epithelial lining, which surrounds the ventral 

 nerve-cord ; and two lateral vessels (Iv and Is} unrepresented 

 in the Choetopods, and perhaps also to be regarded as rem- 

 nants of the coelomic cavit}-. Communications between these 

 longitudinal vessels occur through the medium of smaller 

 vessels ; and in some forms, such as Nephelis, the connection 

 between the lateral and ventral vessels takes place through 

 ampullae, globular vesicles arranged in two pairs on each 

 side of a number of metanieres and receiving blood-vessels 

 from the ventral sinus, while other vessels passing to the 

 main lateral vessels arise from them. In many forms, espe- 

 cially among the Guathobdellidse, a rich plexus of capillary 

 vessels penetrates the hypoderinis. 



The union of the blood vascular system with sinuses which most prob- 

 ably represent portions of the coelomic cavity suggests an intimate relation, 

 so far as its origin is concerned, of the vascular system with the coalom ; 

 and this view is borne out by what has already been seen to occur in the 



