14 



and isolated fibres which terminate in the inner mantle 

 fold. 



Within the mantle there is, at the margin, a filling tissue 

 composed of fine connective tissue fibres, in which ran the 

 muscle bundles and the trunks and finer branches of the 

 pallial nerve plexus ; passing inwards, this begins to 

 include large irregular communicating spaces, and is soon 

 reduced to a mere lining to the epidermal surfaces from 

 which trabecule pass inward, forming a very coarse net- 

 work. The trabecule consist of rather dense fibrous tissue 

 with scattered nuclei. Far back from the edge this 

 becomes reduced to little more than a layer of small 

 nuclei and a few fine fibres. Delicate bridges of fibrous 

 tissue unite the two epidermes, so that the whole cavity 

 in the interior of the mantle lobe is divided into a system 

 of inter-communicating spaces which are generally empty 

 in sections, but are most probably blood sinuses. The 

 inner epidermis is composed of flat squamous cells. Near 

 the point of attachment of the mantle lobe to the body- 

 wall the former becomes much thicker, and the spongy 

 tissue in its interior attains a greater development. 



If now, the mantle lobe being removed, the labial palps 

 be cut away along their attached borders, and the gills be 

 carefully removed by cutting close to their bases, the 

 portion of the body lying between the adductors is laid 

 bare. The base of the ctenidium (Br 1 ., fig. 3) extends 

 downwards obliquely from the region of the body extend- 

 ing up into the umbones, to the lower horizontal level of 

 the posterior adductor. Here the bases of the right and 

 left ctenidia become free from the body-wall, and continue 

 to pass ventrally and posteriorly till their posterior extrem- 

 ities fuse with each other in the middle line, and with the 

 horizontal shelf, referred to above, as formed by the 

 extension inwards of the first fusion of the mantle lobes 



