148 Prof. G. J. Allman on the Anatomy 0/ Actseon. 



but whether this is destined to receive from the oesophagus a por- 

 tion of the ahmentary matter, thus performing the office of a crop^ 

 or whether its function is that of simply discharging into the ali- 

 mentary tract some peculiar product of secretion, I have been 

 unable to determine. 



But perhaps the most remarkable of all the organs accessory 

 to digestion is the singular ramified system which constitutes so 

 large a portion of the viscera of Actceon, and which M. de Quatre- 

 fages, conceiving it to be of high importance, has assumed, along 

 with certain other peculiarities of structure which he supposes 

 invariably to accompany it, as entitling him to unite into a new 

 order a certain number of Gasteropodous MoUusca, distinguishing 

 them under the name of P hleb enter ata. 



The system under consideration consists in our mollusk of two 

 tubes which open, close to one another, into the stomach, at its 

 cardiac extremity. Near to its termination in the stomach each 

 tube divides into two branches, one, PI. NLkk, passing forwards 

 towards the head, the other, / /, running backwards into the late- 

 ral expansions. Each of these branches then undergoes minute 

 subdivision, the ramifications extending forwards nearly to the 

 anterior margin of the head, and backwards to the posterior ex- 

 tremity of the lateral expansions. The ultimate ramuli terminate 

 each in a cul-de-sac, mmm, and PI. VII. fig. 4, filled with a green 

 or brownish substance, in which the microscope enables us to 

 detect oleaginous globules floating through a fluid of aqueous 

 consistence. 



It is to this curious system, visible through the semitransparent 

 integuments, that the prevailing colour of the species must be 

 attributed ; its nature and function, and its claim to constitute a 

 character of ordinal importance, will be presently considered. 



Vascular and Respiratory systems. — The best way of detecting 

 the vascular system of Actceon is to view the mollusk as an opake 

 object by means of a single lens and without compression, when 

 a delicate ramification of vessels will be seen creeping over the 

 upper surface of the foliaceous expansions, PL V. fig. 4. Those 

 vessels which spread themselves over the posterior part of the ex- 

 pansions terminate in two trunks, a a, one for each lateral half, 

 which running nearly parallel to the mesial line and at a short 

 distance from it, at either side appear to end in a circular canal, b, 

 into which the vessels from the anterior part of the expansions 

 also open. This circular vessel cannot be satisfactorily seen with- 

 out actual dissection ; it is placed immediately beneath the inte- 

 guments of the back and over the posterior part of the body, and 

 embraces an organ, c, of an irregularly semicircular figure, into 

 whose structure strong fibres may be seen to enter with a reticu- 

 lated arrangement. Beyond this point my researches have failed 



