ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 401 



The following is Iliering's classification of Mollusca : — 

 MOLLUSCA. 

 Class I. Amphineura. 



„ II. ACEPHALA. 



„ III. Cephalopoda. 



„ IV. SoLENOCONCH\E. 



„ V. COCHLIDES. 



Order 1. Chiastoneura. 



„ 2. Orthoneura. 



„ 3. Heteropoda. 

 „ VI. Protocochlides (Rhodopidae). 

 „ VII. Pteropoda. 



„ VIII. ICHNOPODA. 



Order 1. Nudibranchiata. 



„ 2. Pleurobranchia. 



„ 3. Steganobrancliia. 



„ 4. Branchiopneusta. 



., 5. Nepbroneusta. 

 Classification of Gastropods, based on the Arrangement of the 

 Nervous System.* — Prof. H. do Lacaze-Dutbiers has summed up the 

 results of bis well-known work on various types of Gastropods. In it 

 the " law of connections " has been bis surest guide, and has been most 

 rigorously applied. He has more than once urged that there are four 

 nerve-centres around which all secondary ganglia can be grouped ; three 

 of them are symmetrical — the cerebral, the pedal, and the stomato- 

 gastric. The fourth is asymmetrical, and is always composed of more 

 than two ganglia ; there are often, indeed, as many as five. This asym- 

 metrical centre may be looked upon as the characteristic organ of tbe 

 group, and Prof. Lacaze-Dutbiers considers that its variations afford 

 the best basis for classification. 



When the asymmetric commissure is very short, all the ganglia touch 

 and form an arc which is connected with the brain by a connective equal 

 in length to the cerebro-pedal connective ; in this case the centre is 

 a little below the pedal ganglion, and in front of the digestive tube ; to 

 this type the term gastroneural may be applied ; it is found in terrestrial 

 and aquatic Pulmonata, Gadinia, Onchidium, and Ancylus. 



In the second type the asymmetric centre is divided into two, and 

 occupies a dorsal position ; the connectives uniting the centre are so 

 short tbat they seem to disappear, and all the ganglia appear to have 

 passed to the dorsal side of the digestive tube ; this notoneural type 

 is well marked in Tethys, and is found also in Tritonidae, Dorididee, 

 Ombrellidse, and JEolidiae. 



In the pleuroneural type, which is seen in Aplysia, Bulla, and Philine, 

 the asymmetrical ganglion lies at a distance from the rest, and on the 

 right side ; in the strepsineural condition there is further torsion of the 

 connectives, and this torsion may come from the dorsal (aponotoneural) 

 or from the pedal (epipodoneural) surface ; of the former the Pectini- 

 branchiata, and of the latter the Fissurellidae and Haliotida? are types. 

 The latest classification of the Mollusca is then : — 



I. Astrepsineura. — 1. Notoneura; 2. Gastroneura ; 3. Pleuroneura. 

 II. Strepsineura. — 4. Aponotoneura ; 5. Epijiodoneura. 



* Comptes Remlus, cvi. (1888) pp. 716-24. 

 1888. 2 F 



