26 THE VOYAGE OF H.M.S. CHALLENGER. 



pallial, posterior apertures are surrounded by a common ring of tentacles ; the anal 

 aperture is prolonged in a short siphon ; the branchial aperture has a large internal 

 valve ; finally, there is a very large anterior pair of labial palps, and a small 

 posterior pair. 



In Cusindaria the characteristics are different and very special. There are four 

 separate pairs of apertures on the branchial partition ; the two pallial posterior orifices 

 are prolonged by siphons united throughout all their length ; the labial palps are all 

 much reduced. 



We may therefore classify the three genera as follows : — 



I. Apertures of the branchial partition arranged in groups. 



1 . Two groups of apertures on each side Poromya. 



2. Three groups of apertures on each side Silenia. 



II. Separate apertures, to the number of four pairs Cuspidaria. 



For the first two genera we may employ the family name created by Dall, 

 Poromyida3 ; for the third we must form a special family, Cuspidariidse, taken from the 

 same author. 



B. Phylogeny. — We have seen that a " branchial " partition pierced with apertures, 

 so that the two pallial chambers communicate with each other, is a common character- 

 istic of the three genera we have been considering. But, if we compare them with each 

 other, we shall easily discover different successive stages of retrogression in the branchial 

 apparatus. 



In Lyonsiella, a genus l)elonging to the group Anatinacea (sensu latiorc), the gills 

 are also united to the mantle along their entire length, to each other behind the foot, 

 and to the division between the two siphons. They thus form two great pallial 

 chambers, corresponding exactly to the two chambers of the Septibranchia. Here, 

 then, we have the starting-point of the strange arrangement which characterises tliis 

 latter group. 



But in Lyonsiella cd>yssicola (see PI. II. fig. 10) the structure of the gills is 

 preserved, and the branchial lamellae cover the partition from the anterior adductor to 

 the division between the siphons. 



The first stage of reduction is seen in Poromya, in which the partition has 

 already become muscular, but has retained two groups of lamellae on each side (PI. III. 



%• 7). 



The reduction is still greater in " Silenia," in which the plates have disappeared, 

 and only form the lips on each side of the apertures in the three pairs of groups 

 (PI. III. fig. 10). 



The reduction is extreme in Cusjndaria, in which there are only four pairs of 



