MORPHOLOGY OF MELIBE 



bodies are located, hence the visceral, or perivisceral, cavity. 

 The pericardial chamber, discussed below, is a true coelom. 



6. The Alimentary Canal. 

 (1) The Buccal Cavity. 

 As in other organisms the buccal cavity of M . leonina is the 

 beginning of the alimentary canal. It corresponds very closely 

 with Alder and Hancock's (1864) and Eliot's (1902) descriptions 

 of M. fimbriata. The mouth is bounded by the two 

 lateral, slightly furrowed lips (PI. 27, fig. 1, L, PI. 29, fig. 17, M, 

 PI. 32, fig. 35) ; the furrows are not seen in fig. 1 as they are 

 smoothed out by the swelling of the lips, but they may be seen in 

 fig. 35 which is a photograph of a transverse section through the 

 buccal cavit}?". Within the mouth there is a uniform invagina- 

 tion of the ectoderm (fig. 35), and this invagination pro- 

 duces a number of folds or corrugations which increase in 

 depth and finally merge with those of the oesophagus. The 

 food, as it is engulfed, passes directly through the oesophagus 

 into the proventriculus. No masticatory process is carried 

 on in the mouth for the simple reason that this species is 

 absolutely void of tongue, radula, or mandibles. The food is 

 swallowed whole, as evidenced by the contents of the alimentary 

 canal including the intestine, and is disintegrated by the 

 digestive processes only. The jawdess condition of this species 

 is a character common with that of its relative, Tethys 

 Linnaeus. Jeffreys says: 'Tethys has neither jaw nor 

 tongue.' And Vayssiere (1901) finds for Tethys fimbriata 

 (S. leporina) : 'Buccal bulb absent. Large anterior 

 chamber ; having exterior circular muscle-fibres probably 

 used in mastication. (Cette region ofTre a sa surface exterieure 

 un anneau ^musculaire, auquel correspond interieurement un 

 anneau de plis longitudinaux presque tendineux, que Ton pent 

 considerer comme un organe masticateur.) ' However, this 

 jawless condition does not prevent Tethys or Melibe from 

 being carnivorous, as is shown by the contents of their stomach 

 (von Jhering, 1876 : 37 ; Berg, 18S2 ; Vayssiere, 1901 : S4-5 • 

 Eliot, 1902: 69; Agersborg. 1919 : 272; 1921: 228,232). 



