ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 205 



medium. Sometimes the coral covers in the mouth of the tube, and 

 then the worm perishes : this, however, seems to happen very rarely. 

 The presence of the worm causes an abnormality in the form of the 

 coral, which, when alone, retains throughout life the discoid form of 

 the young. Pontes is another example of a coral with which worms 

 live and its interior may be often seen to be perforated with worm- 

 tubes. 



MoUusca. 



General Account of the MoUusca.* — E. Eay Lankester has an 

 exhaustive article on the MoUusca, which he arranges as follows : — 



Phylum MoUusca. 

 Branch A. Glossophora. Branch B. Lipocephala. 



Class 1. Gastropoda. Class 1. Lamellibranchia. 



Br. a. Isopleura, e. g. Oyster, Mussel, Clam, 



e. g. Chiton, Neomenia. Cockle. 



Br. 6. Anisopleura, 



e. g. Limpet, Whelk, Snail, Slug. 

 Class 2. Scaphopoda, e. g. Tooth-shell. 

 Class 3. Cephalopoda. 



Br. a. Pteropoda, e. g. Hyalsea, Pneumodermon. 

 Br. h. Siphonopoda, e. g. Nautilus, Cuttles, Poulp. 



After a general account of the MoUusca as a phylum of the 

 Coelomata, the author describes a " schematic mollusc " ; it has a 

 head on which are placed a pair of short cephalic tentacles ; the aper- 

 tures of a pair of nephridia are seen to the right and left of the anus ; 

 the most characteristic organ is the foot (podium) which is probably 

 genetically connected with the muscular ventral surface of the 

 Planarians, and with the suckers of Trematoda. On the dorsal surface 

 is the visceral hump or dome, protected by a shell, which is single, 

 cap-shaped and symmetrical ; the integument of the visceral dome 

 forms a primary shell-sac or follicle. The wall of the body forms a 

 flap or skirt — this is the mantle. Underlying this are the ctenidia or 

 gill-combs, to which it is well to give a non-physiological name. 

 Near the base of the stem of each ctenidium is a peculiar patch of 

 modified epithelium, which tests the respiratory fluid and is persistent 

 in its position and nerve-supply throughout the MoUusca ; it is the 

 olfactory organ of Spengel and may be definitely known as the 

 osphradium. The term " gonad " is applied to the ovaries or spermaries, 

 and it is pointed out that, at present, we cannot say whether the 

 gonad was primitively median, or paired. The disposition of the nerve- 

 cord is highly characteristic. A general sketch of the phenomena of 

 development follows. 



The systematic review commences with pointing out the import- 

 ance from a classificatory point of view of the radula. The Isopleura 

 are divided into the Polyplacophora (Chitons), Neomeniae, and Chseto- 

 derma ; the two latter must be associated with the Chitons now that 



* Ency. Brit., xvL (1883) pp. 632-95 (152 figs.). 



