2-M -TERRA NOVA" EXPEDITION. 



arc really nothing more than local concentrations of the ordinary muscles of the body- 

 wall, lie further bark and pass, from their origin upon the .shell on a level with the 

 oesophagus, dmvn the surface of the visceral mass on either side of the pedal ganglia, 

 into the base i if the byssal papilla. 



Flint. This is of the type commonly met with in the Arcida;, laterally com- 

 pressed, with prominent toe and heel and with the ventral surface cleft by a deep byssal 

 groove into which protrudes a tongue-shaped papilla (Fig. 1, %./'.), beset upon its 

 surface by a series of low longitudinal laminae. The byssus consists of several (three in 

 the specimen examined) short flattened filaments. 



Nervous System and Sense- Organs. The cerebral and visceral ganglia are respectively 

 widely separated, the pedal coalesced. Abdominal sense-organs and otoeysts are well 

 developed and occupy the usual positions. 



In addition to these sense-organs there is, upon the anterior extremity of each 

 inner gill-lamella, where it merges with the hinder end of the inner labial palp, a well- 

 marked ' cephalic "eye similar to those described by Pelseueer (11. 16) audThiele (19, 

 p. 380) in certain Mytilacea and Arcacea (Figs. 1, 3, o'.).* The eye consists of a deeply 

 pigmented open cup (Fig. 4) occupied by a homogeneous cuticular plug. The nerve- 

 supply to the organ was not traced. 



It was observed by Pelseneer (16, p. 77(i) that among Mytilidse and Arcidse, 

 " cephalic " eyes, though common, did not occur in species inhabiting a depth of more 

 than 250-300 metres a depth to which light does not penetrate. In view of this 

 observation, which is of some importance in estimating the probable functional value 

 of these organs, it is of interest to find that the present specimens of Lis#<tmi were 

 obtained from a depth of 250 fathoms, those of Philobrya (in which also "cephalic" 

 eyes occur) from 160 and 250 fathoms, and those of Ailm-nnrni from depths ranging 

 from 50 to 250 fathoms. 



Diijexth-i' Orijans. The lips and palps are ribbon-like as in other Areas. The free part 

 of the outer palp is for most of its extent very narrow (Fig. SB, oj>!p.) and represents 

 only the extreme margin of the organ, its basal parts being adherent to the mantle. 



Mesial to the inner lip, in the angle between the lip and the body-wall, and at a 

 short distance behind the mouth, there is on either side a pore that leads into a finger- 

 shaped caecal tube (Figs. 1, SA, and SB, I.e.). The caecum (Fig. 6, I.e.) is lined by a 

 deep columnar epithelium, and is occupied by a homogeneous material, probably in the 

 nature of a secretion. 1 have not come across any description of similar caeca in this, 

 position in other Lamellibrauchs, but in the present collection I have observed them, as 

 will be mentioned later, in Adacnarca and Philobrya, There seems little doubt that 

 these tubes arc glandular, but what their purpose may be, and with what organs in 

 other groups of Molluscs they may be compared it is difficult to suggest. 



* Cephalic eyes have been recorded in Mylilus, Modiolaria, Llthiiiliumiii, Aeli-ulti (i.tr^iiltini, 

 i us, Mt'li-<ii/r!iia, Ainnniii, Pvrnu, Area lactea, Area spp. 



