Mycetoinori)ha. 581 



The Mantle Cavity and its relation to the 

 Visceral Mass. 



When tlie small visceral mass is first seen in relation to the 

 mantle cavity it is realised how considerable is the development of 

 the mantle cavity compared with other Ehizocephala. The diniensions 

 are further increased by the lateral divei-ticula which coninnmicate 

 (Fig. E) with the niain cavity, and which serve the purposes of 

 secondary bi-ood ponches. The great development of the mantle 

 cavity is dne to the great numbers of larvae which are produced as 

 well as to the large size of the larvae themselves. At the time of 

 capture the entire cavity was tightly i)acked with the Cypris forms, 

 but in the sections as drawn for this paper they are omitted in 

 the central cavity. 



The mantle (cf PI. 16 Fig. 5) is remarkable for its thinness. 

 There is externally a cuticle of moderate thickness (but as thick as 

 the whole of the rest of the mantle) secreted by an irregulär ecto- 

 dermal epithelium (ecf) with well marked nuclei but no definite cell 

 boundaries. This is succeeded by a layer of muscle fibres (mu), 

 which are mostly longitudinal in arrangement. The musculature is 

 especially developed round the apertures of the lateral diverticula 

 into the mantle cavity. Lastly there is a thin layer of internal 

 ectoderm (ect^) lining the mantle cavity, with a very thin cuticle. 

 Between the internal ectoderm and the muscle layer are to be found 

 the bodies which I regard as eggs retarded in development. 



The mesenteries. The main attachment of the visceral 

 mass to the mantle is effected by a very thin lateral mesentery, 

 running on the curved left side of the body (cf. Fig. A, E, F. G etc.). 

 But in front of the apex of the bay and in the neighbourhood 

 of the mantle duct an extensive fusion of the visceral mass with 

 the mantle has taken place both dorsally and ventrally. A small 

 diveiticulum of the mantle cavity is however found in sections through 

 this region (Fig. D) intruding between the visceral mass and the 

 under surface of the mantle and this increases in extent when 

 traced forward, until in the neighbourhood of the internal aperture 

 of the mantle duct the visceral mass is only supported by a wide 

 dorsal concrescence with the mantle. Further forward the connection 

 ceases altogether, 



T h e c n n e c t i n f t h e i' o o t s y s t e m with the visceral 

 mass (cf. Fig. J). The rootsystem passes through the peduncle by 



