28o ox THE VASCULAR SYSTEM OF THE LOWER ANNULOSA 



The smaller branches are all richly ciliated. In other Distomata the 

 lateral trunks appear to be converted into excretory organs, as they 

 are full of minute granules ; they remain eminently contractile ; but 

 their connection with the system of smaller ramified vessels ceases to 

 be easy of demonstration. As Van Beneden and others have shown, 

 they still form one system ; but the cilia are no longer to be found in 

 the smaller ramified vessels, having sometimes vanished altogether ; 

 at others, being discoverable only here and there in the minute 

 ultimate terminations of these A'essels. In certain Nematoidea the 

 vascular system is reduced to a couple of lateral contractile vessels, 

 altogether devoid of cilia, but communicating, by a small aperture, 

 with the exterior. Now there is no doubt that, in all these cases the 

 "vascular system" is physiologically a respiratory and, perhaps, 

 urinary system ; while the common cavity of the body represents the 

 blood-vascular system of the Mollusca and Articulata. However, 

 Echinorhynchus possesses a vascular system of the same nature as 

 that of a Nematoid or Distomatous worm, but presenting no cilia, and 

 having no external opening ; thus forming a closed vascular system, 

 homologous with those previously described, and differing from them 

 onl)' in the fact of its closure. But from hence it is a very easy and 

 natural transition to the vascular system of the Annelida ; and the 

 author stated his conviction, based not only upon these, but upon 

 man)' additional reasons, that these so-called blood-vessels, and those 

 of the Echinodcrmata, form, in fact, only the final term of a series, of 

 which the so-called water-vascular system of the Rotifera constitutes 

 the commencement. If, however, these vessels have really nothing to 

 do with the proper blood-vascular system of the higher Annulosa, 

 with what system of organs are they homologous .? In answer to 

 this question, the author stated his belief that they correspond with 

 the trachea; of Insecta, which present a similar extensive ramified 

 distribution, and, in some cases, as in the larvae of the Libellulidae, 

 constitute as complete!}? closed a system of vessels. 



