484 THE ANIMAL KINGDOM. 



(4), xii. 1859. Sucker of Natantia = Heteropoda, Niemiec, Recueil Zool. Suisse, ii. 

 1885. 



Nerve eminences of Fissurella, &c., gustatory organs , Haller, M. J. ix. 1883, 

 pp. 44, 73. Tactile bodies on tentacles, Flemming, A. M. A. xxiii. 1884. 



Eyes of Onchidium, Semper, Reisen im Archipel. d. Philippinen, (2), iii. 

 Erganzungsheft, 1877, and A. M. A. xiv. 1877. 



Histology of nervous system, Haller, M. J. ix. 1883. 



On formation of ctenidium, Osborn, Studies from Biological Laboratory Johns 

 Hopkins University, iii. pt. i. 1884 ; of Fasciolaria, Id. ibid. pt. 3, 1886. 



Nephridia-, structure, in marine Rhipidoglossa, M. J. xi. 1886; of Patella, 

 Cunningham, Q. J. M. xxiii. 1883; of ' Aplysia, Id. Mittheil. Zool. Stat. Naples, iv. 

 1883. See a note on function, Joliet, C. R. 97, 1883. 



Two forms of spermatozoa, Von Brunn, A. M. A. xxiii. 1884 ; Z. A. vii. 1884. 



Mimicry in Mollusca, Osborn, Science, vi. 1885. 



BRANCH II. LIPOCEPHALA. 



Mollusca with rudimentary prostomium. Eyes absent on the prosto- 

 mial region of the adult. No odontophore or jaws. 



CLASS LAMELLIBRANHCIATA. 

 (Conckifer a; Pelecypoda ; Elatobranchia). 



Lipocephala with very well-marked bilateral symmetry. There are two 

 large mantle-folds, a right and left, each covered by one of the valves of the 

 shell. An elastic ligament unites the two valves of the shell together in the 

 median dorsal line. The foot is laterally compressed and tisually sharp-edged. 

 There are two pairs of labial tentacles. The gill-filaments usually undergo 

 concrescence and form gill-lamellae. There are three pairs of ganglia, the 

 cerebro-pleural, pedal, and visceral. The liver, auricles, nephridia, sexual 

 glands are paired and symmetrical. Salivary glands and accessory organs 

 of generation are wanting. 



The valves of the shell are usually equivalve, but rarely equilateral 

 as in some species of Pecten. Accessory pieces may be present in the 

 median dorsal line as in Pholas. The valves may be small and cover the 

 mantle-surface incompletely, and then the animal clothes its burrow in 

 wood (Teredo) or sand (Septarid) with a calcareous lining. Or though 

 free originally, they become fused at an early period into a continuous 

 calcareous tube secreted by the general surface of the mantle, open pos- 

 teriorly, and often anteriorly as in Aspergillum and its allies (Gastrochae- 

 nidae). In Ostreidae the right valve is firmly fixed to stones. The shell 

 is often produced into processes or ridges formed by lobes of the mantle- 

 edge ; it is generally coloured by pigments derived from unicellular 

 glands in the same region. The ligament must be regarded as a median 



