36 Moll 
MOLLUSCA. 
Nassa and Buccinum the differences are slight; in the Pur pur idee the 
siphon is shorter than in the Buccinidce, the crop well-developed, the 
special gland solid, and the stomatogastric cerebral centres closely 
united. Concholepas is a Purpura , modified by atrophy of'the posterior 
lobe of the foot, and the development of an enormous anterior lobe, in 
consequence of which the viscera have acquired a dorsal position. Bou- 
vier (49). 
8 . Organs of Sense. 
Moseley (273) shows that two kinds of tactile organs (megalassthetes 
and miersesthetes) are present in the shells of certain species of Chiton- 
idee, and that well-developed eyes which are on the type of Helix , not of 
Onchidium or Pecten , occur as a modification of the former. 
Brief summary of knowledge of sense-organs ; Scharff (322). 
Cephalic appendages of some Pteropoda have been examined by Pel- 
seneer (285). Clione has two pairs of tentacles, the posterior bearing 
eyes, and three pairs of buccal cones ; these do not bear suckers, but 
sensory organs, which may be appropriated to a special sense interme¬ 
diate between that of taste and smell. They contain a mass of glandular 
cells, whose secretion overspeads their surface. Clionopsis has two pairs 
of tentacles, but no buccal cones. Pneumodermon also has two pairs of 
tentacles, and instead of buccal cones two acetabuliferous appendages. 
Regarding the structure of the suckers the author agrees in the main with 
Niemiec (278). Compare also Wagner (373). 
Hilger (182) has investigated the eyes of over 70 species of Gastro¬ 
pods, and divides them into two groups:—(1) In the lowest Prosobran- 
chiata , Cyclobranchiata , Aspidobranchiata , &c., the eye is an open cup, 
though in some instances traces of what may have been a covering are 
found. (2) The eye is a space closed anteriorly by a “ pellucida,’ 5 and 
containing a lens or vitreous mass, or both. The optic nerve contains 
ganglion cells in all Prosobranchiata , other groups presenting variations 
in this respect. In contradistinction to Fraisse, the author observed the 
optic nerve in Patella. The retina consists of two kinds of elongated 
nucleated cells, pigmentless and pigmented; they are disposed in groups, 
one of the former being surrounded by a number of the latter. Upon the 
layer formed by these rests one of clear rods, into the axis of each of 
which the end of a pigmentless cell projects. The cornea (“ pellucida ”) 
consists of an inner and outer epithelium and intermediate connective 
tissue, which varies in thickness in different species. Lens and vitreous 
are commonly both present, but either or both may be absent ; the last 
case is apparently seen in the Cyclobranchiata , though Hilger is disposed 
to attribute this to defective preservation. The vitreous is wanting in 
Stylommatophora, in which the rods rest directly upon the lens. The 
lens appears to be structureless in adults, but shows concentric layers in 
embryonic forms. 
Butschli (61) gives a comparison between the results of this paper 
and those of Grenacher, regarding the Cephalopod retina. 
Carriere (67) finds that in most Gastropods the cornea consists of a 
thin layer of connective tissue, covered with epithelium, and lined by a 
