8 THE VOYAGE OF H.M.S. CHALLENGER 



7. Fossarus cereus, Watson. Station 184; 1400 fathoms. 



This animal does not possess cervical lobes ; it is therefore unlikely that it is 

 a true Fossarus. Eyes are entirely wanting. 



FlSSURELLID.^. 



8. Puncturella hiychia, Watson. Station 47 ; 1340 fathoms. 

 As in the preceding forms, there are no visible eyes. 



11. PELECYPODA, 



The internal structure of the Pelecypoda does not generally vary much in its 

 essential features (alimentary canal and its adjuncts, circulatory and excretory apparatus, 

 genital organs, nervous system). On the other hand, the external organs, such as the 

 gills, the mantle, the foot, having more direct connection with the surrounding medium, 

 are much more variable in form, because of the immediate action of external conditions, 

 and because of their adaptation to the diverse life conditions of the Pelec^'poda. 



We shall therefore devote special attention here to the form and modification of 

 these latter organs. 



Arcacea. 



Four different genera of this group are represented in the collection entrusted 

 to me : MaUetia, Yoldia, Limojms, Area. 



1. Malletia 2^ctUida, ^m\t\\. Station 137; 2550 fathoms. 



A single specimen slightly damaged. 



The mantle is quite open, except behind, where its internal fold forms two siphons 

 joined together. In the anterior portion the mantle is not separate from the visceral 

 mass, and the visceral glands extend to it (PI. I. fig. 7, e). 



The siphons are not separate from each other. The ventral one (inhalent), which is 

 the shorter, is open ventrally all its length ; it does not form a closed tube, like that 

 of the other Pelecypoda with two siphons, but resembles the rudimentary open branchial 

 siphon of Modiolana nigra, and the funnel of the Nautilus. In Yoldia (see later 

 Yoldia isonota) this inhalent siphon forms a completely closed tulje, like that of the 

 other Pelecypoda with two siphons. 



This open siphon cannot be compared to the open pseudo-siphon of Led a.^ The 

 ' Desliayes, IJistoire Naturelle des MoUusques (Exploration Scientifique de I'Algerie), pi. cxv. fig. 2, c. 



