230 INVERTEBRATE MORPHOLOGY. 



of longitudinal and circular fibres ; and in addition, between 

 these, a layer composed of fibres wliich cross one another 

 obliquely is usually present. A marked distinction from what 

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

 Hirudinea is traversed by a parenchyma, recalling that of the 

 Platyhelminths, 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, Cd) 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 ccelom — 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 Chse- 

 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 Chsetopods, and perhaps also to be regarded as rem- 

 nants of the coelomic cavity. Commu-nications 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 metameres 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 GnathobdellidsB, a rich plexus of capillary 

 vessels penetrates the hypodermis. 



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

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

 so far as its origin is concerned, of the vascular system with the coelom • 

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



