THE GASTROPODA 175 



is extremely reduced. The phylogenetic relationships between the Nudi- 

 branchs and Tectibranchs are clearly exhibited by the organisation of the 

 Pleurobranchomorpha, in which there is neither a pallial cavity nor an 

 osphradium, and the respiration is largely pallial. The shell is altogether 

 absent in Pleurobranchaea ; the nervous system is concentrated on the 

 dorsal side of the oesophagus ; spicules and cnidocysts are found in the 

 dorsal integuments. The mantle of the Doridomorpha is completely 

 homologous with that of the Pleurobranchidae, and is in no wise to be 

 regarded as an epipodium reflected over the dorsal surface of the body. 



Fid. 159. 



gastric ganglion. 



TRIBE 1. TRITONIOMORPHA. 



Nudibranchia in which the liver is wholly or partially contained in 

 the visceral mass. The anus is lateral, on the right side. There are 

 generally two rows of ramified dorsal appendages (Fig. 83, II). The genital 

 duct is diaulic ; the male and female orifices contiguous. FAMILY 1. 

 TRITONIIDAE, Adams. The anterior tentacles form a frontal veil ; the 

 foot rather broad. Genera Tritonia, Cuvier ; stomach without horny 

 plates ; British (Fig. 83). Marionia, Vayssiere ; stomach with horny plates. 

 FAMILY 2. SCYLLAEIDAE, Alder and Hancock. No anterior tentacles ; 

 dorsal appendages broad and foliaceous ; foot very narrow ; stomach with 

 horny plates. Genus Scyllaea, Linnaeus ; pelagic. FAMILY 3. PHYLLIR- 

 HOIDAE, Adams. No anterior tentacles and no dorsal appendages ; body 

 laterally compressed ; transparent natatory forms. Genus Phyllirhoe, 

 Peron and Lesueur (Fig. 161). FAMILY 4. TETHYIDAE, Alder and 

 Hancock. Head broad, surrounded by a funnel-shaped velum or hood ; 

 no radula ; dorsal appendages foliaceous. Genera Tethys, Linnaeus ; 

 foot broad; no mandibles (Fig. 160, B). Melibe, Bang ; foot narrow; 

 mandibles present. FAMILY 5. DENDRONOTIDAE, Alder and Hancock. 

 Anterior tentacles forming a scolloped frontal veil ; dorsal appendages and 

 tentacles similarly ramified. Genera Dendronotus, Alder and Hancock ; 



