vii MOLLUSCA HYPOBRANCHIAL GLAND, HEAD 101 



VII. The Hypobranehial Gland. 



(Slime gland of the Protobranchia, epithelial shield of the Ptcropoda, 

 etc., anal gland, etc.) 



This is an organ very commonly found on the molluscan mantle, 

 always occurring near the ctenidium, at its base or between it and 

 the rectum. Cf. on its position and occurrence Section V. 



The hypobranchial gland varies considerably in shape, but is 

 never a multicellular, acinose, or tubular gland wifch efferent ducts. It 

 is originally a more or less extended area of the epithelium of the 

 mantle cavity (generally of the inner surface of the mantle) in which 

 epithelial glandular cells are particularly numerous. In this condition 

 it is not very distinct from the parts around it, but it may become 

 more definitely localised, and may assume a definite shape ; and in 

 this latter case, the glandular epithelium of which it consists may also 

 become folded in order to obtain a larger secretory surface, the folds 

 being more or less closely crowded together and projecting into the 

 mantle cavity. This gland often secretes a large quantity of mucus. 

 The purple gland of certain Prosobranchia (Pwpwra, Murex, Mitra) is a 

 hypobranchial gland, the slimy secretion of which is, immediately 

 after ejection, colourless or only slightly coloured, but under the 

 influence of light becomes violet or red. In Purpura, the gland consists 

 of two parts which differ slightly in structure. 



VIII. The Head. 



If by the word head is meant an anterior portion of the body 

 more or less distinct from the rest, possessing a mouth and specific 

 sensory organs, the Lamellibranchia must be considered headless, and 

 as such have been distinguished as Acephala from other Mollusca. 

 This absence of a head in the Lamellibranchia cannot be regarded as 

 a primitive condition, 1 but is to be accounted for by their general 

 habit of living in mud, and by the strong and peculiar development 

 of the mantle and shell, which, by cutting off the anterior portion of 

 the body (with the mouth) from direct contact with the outer world, 

 renders specific sensory organs useless. In those Molluscs which 

 have to seek, seize, and crush their food, a projecting head carrying 

 sensory organs and furnished with buccal armature is of great use. 

 Bivalves, however, feed on particles brought to the mouth by the 

 water which by the motion of cilia is driven through the mantle 

 cavity ; buccal armatures are thus unnecessary. 



In the Cephalopoda, the head is strengthened by the incorporation 

 with it of the foot, here transformed into a circle of arm-like prolonga- 



1 Hence the term " Lipocephala, " suggested by Lankester. 



