506 JOSEPH STAFFORD, 



the first question it is probable that, notwithstanding a well developed 

 intestinal apparatus, the animal absorbs nutritive fluids through its 

 external surface as nearly related Cestodes without any intestine do. 

 As regards the second, there might be the same reason for a sym- 

 metrical arrangement of these as of other organs. 



Taking into account the number of skin glands and the degree 

 to which they are generally distended with secretion, each animal is 

 capable of producing a considerable quantity of secretion fluid, and in 

 cases where, as I have found, as many as twenty to thirty (even fifty 

 or ninety-eight) Aspidogasters are living in a single Anodon, if this 

 secretion is of a poisonous or even irritating nature, it must have a 

 great influence on the metabolic processes of the host, for it is ex- 

 truded immediately into the blood of the mussel. The parasites them- 

 selves are to some extent bathed in their own secretion and must 

 carry some of it into their intestine. 



Excretory system. 



The excretory system of Aspidogaster is one of the most beautiful 

 to be found in any animal and one of the most interesting in the 

 group of the Trematodes. Its brightly glancing canals of varying 

 diameter and straight, wavy, or spiral course must have enlisted the 

 attention of its numerous observers from their discovery to the present 

 time. In 1832 Nordmann described having seen the blood pass for- 

 wards and backwards in separate vessels which he took for arterial 

 and venous vessels, v. Siebold, in 1837, distinguished two systems 

 of vessels — one closed, much branched, and one opening out at the 

 foramen caudale. He discovered also the ciliary organs on the inner 

 walls of the vessels: "Längslappen, deren lange freie Ränder man 

 wellenförmig schwingen sieht, wodurch man leicht in Versuchung ge- 

 räth, zu glauben, es schlängelten sich fadenförmige Würmer in den 

 Gefässen". Dujardin (1845) assigned to them a double function — 

 circulatory as well as respiratory. 



AuBERT and Huxley have given general descriptions of this 

 system but owing to the greater imperfections of the methods and 

 appliances of their time (1855 — 185G) these accounts require now to 

 be succeeded by more accurate ones. Voeltzkow in comparatively 

 recent time described the system at a whole — so far as known to 

 him, but his account still contains many imperfections, chief of which 

 have relation to the position of the large collecting vessels, their 

 external opening and anterior continuation into the smaller vessels, 



