390 Dr. 0. A. L. Morch on the Systematic Value of the 



Thus the liver is strongly arborescent in animals having a dermal 

 respiration (Pellibranchiata, Gymnobranchiata), and becomes 

 more and more compressed in proportion as the gills are deve- 

 loped (ScyllaA, Pleurophyllida) ; but it is most complicated in 

 Mollusca covered with a shell. The same is the case with the 

 renal organ, which becomes rounded and spongy, as is to be 

 seen in the plates of Mr. Hancock's excellent paper* on the renal 

 organ of nudibranchiate Mollusca, and in the work of Gegenbaurf 

 on the Heteropoda and Pteropoda. The compact structure of 

 this organ in the testaceous Mollusca may be seen in many of 

 the plates to the ' Voyage de Y Astrolabe.' 



In the Pellibranchiata the generative glands partake of the 

 arborescent form of the liver and kidney : thus in Elysia %, &c, 

 the albuminous gland, the hermaphrodite gland, and "laglande 

 en trefie" of Moquin§ are strongly arborescent. 



The dorsal plates of Placobranchus and the arborescent tufts 

 of Dendronotus and Onchis may be considered the most imper- 

 fect forms of gills. The hepatic papillae of the JEolidida are 

 probably not homologous with the branchial leaves of Gastero- 

 poda (e. g. Ianthina, Pterotrachcea, Doris), as these organs in 

 Bornella are found simultaneously with arborescent tufts of the 

 skin, which may be considered homologous with the branchiae 

 of the Doridida. There appears to be a gradual transition in 

 the respiratory organs of Tritonia, Hcptabranchus, Hexabranchus, 

 and Lamellidoris. It must, however, be remembered that, ac- 

 cording to Dr. Hancock, the gills of Doris receive hepatic blood. 

 The gills in symmetrical animals are generally situated on each 

 side of the body, e. g. Acephala, Cyclobranchia, Inferobranchia. 

 In asymmetric spiral testaceous Mollusca the single gill becomes 

 smaller, and is said to be reduced to a filiform undulated vessel, 

 as in Vermetus and Onustus ||. I have observed, however, in a 

 specimen of O. trochiforrnis, that this vessel opens through a 

 pore on the outer side of the mantle opposite to the shell ; it 

 may perhaps be in some relation with the renal organ. 



The insignificance of the gills as a systematic character is 

 evident by comparing the Heteropoda, from the entirely gill- 

 less Firoloides and Pterotrachaa with external gills, to At- 

 lanta exhibiting perfectly internal gills. The same fact is to be 

 seen in the following series of allied genera : — Stylochilus, No- 

 tarchus, Aplysia, Bulla, &c, Actaon, Odostomia, and Obeliscus. 



* Trans. Linn. Soc. 1864. 



t Untersuchungen iiber die Pteropodeu u. Heteropodcn, 1854. 

 X Journal de Coiiebyliolo»ie, vol. i. 



§ " Le talon" (St. Simon) = "petite glande secre"teur" (Souleyet, /. c. 

 t. (i. f. 5). 



|| Morch, Journal de Conchyliogie, vi. p. 308. 



