286 MR T. J. EVANS ON 



gill, since it is a progressive process in the group and is incipient in many Tectibranchs, 

 including Aplysia. Moreover, at least one Dorid, Trevelyana crocea, has a single un- 

 divided laminate gill indistinguishable from a ctenidium. Furthermore, the three residual 

 units of the pallial complex namely, the kidney and its pore, the intestine with the anus, 

 and the gill occupy in the Dorids just those relative positions which they would occupy 

 had they been turned over from the Tectibranch position into the median dorsal line. 

 Here one is inclined to ask what is the nature of the great cavity, cut off from the 

 underlying hsemoccele, which lies under the dorsal integument of the Dorid, but is 

 absent in all other Nudibranchs. An exactly similar cavity in Pleurobranchus or 

 Oscanius contains a shell-remnant, and is the shell-cavity. In the absence of any 

 information regarding the metamorphosis of the veliger of either Dorids or Pleuro- 

 branchids, it is difficult to find any satisfactory reason for contradicting the homology 

 of these two spaces. 



It is on the above grounds proposed to define the Dorids as ctenidiate Opisthobranchs 

 that have retained the shell-cavity and in which the elements of the pallial complex 

 have moved dorsally into the median line. In this position the ctenidium has under- 

 gone progressive modification within the group, the retractile circlet being its highest 

 development. 



In Tritonia, on the other hand, the residual members of the pallial complex have 

 remained in a more anterior position than they occupy in many Tectibranchs, and in 

 that position the old molluscan gill has been lost. Whereas in the Dorids and Pleuro- 

 branchids the connection of the auricle with lateral integumentary sinuses is supple- 

 mentary to the ctenidial connection, in Tritonia it is the sole remaining connection of 

 the auricle with respiratory sinuses. As a primitive actenidiate animal, however, 

 Tritonia retains many common features with the Dorids and Pleurobranchids. its 

 nearest ctenidiate relatives. 



BIBLIOGRAPHY. 



(1) BERGH, R., "Report on the Nudibranclriata," " Challenger" Expedition, vol. x., 1884. 



(2) " Nudibranchiate Gasteropoda," Danish " Ingolf" Expedition, vols. ii.-iii., 1900. 



(3) DRETER, T. H., " Uber das Blutgefass- und Nervensystem der Aeolididaaund Tritoniadae," Zeitschr. 



f. wiss. Zool, Bd. xcvi., 1910. 



(4) ELIOT, Sir CHARLES, A Monograph of the British. Nudibranchiate Mollusea, part viii., Kay 



Society, 1910. 



(5) " Report on the Mollusea Nudibranchiata collected by the Discover//," National Antarctic 



Expedition Reports, 1907. 



(6) GUIART, J., "Les Mollusques Tectibranches," Causeries scientifiques de la Soc. Zool. de France, 1900. 



(7) - - Contribution a I'etude des Gasteropodes Opisthobranches, Lille, 1901. 



(8) HANCOCK, "On the Structure and Homologies of the Renal Organ in the Nudibranchiate Mollusea," 



Trans. Linn. Soc. Land., xxv., 1865. 



(9) HANCOCK and EMBLETON, "On the Anatomy of Doris," Phil. Trans., London, 1852. 



(10) HECIIT, K., "Contribution a I'etude des Nudibranches," Mem. Soc. Zool. de France, vol. viii., 1895. 



(ROY. soc. EDIN. TRANS., VOL. L., 208.) 



