48 



avicularia which occur in ThalamoporelUi lioticho (PI. VI), Thai, novae boUandiae 

 (PI. VI a), Cribrilina fiyiilaris etc. On the other hand, a cryptocyst is quite lacking 

 in the heterozooecia in the families liicellariulae, Scnipocellariidae, Catena- 

 riidae etc. 



On account of the free movement recjuired by the operculum (mandible) of 

 the heterozod'cia, this is always simple (pag. ;?S), and naturally ends in a 

 straight proximal edge. While the basal and frontal wall of the vestibulum in 

 an ordinary zooecium are connected by two free lateral walls, which on the 

 closing of the operculum are folded, the latter are absent in a helerozooecium, 

 and the vestibulum is consequently here developed in the shape of two 

 separate laminje of which the basal takes up the opercular area, while the frontal, 

 which proximally is joined to the basal, extends over the internal surface of the 

 mandible. We saw above that in an ordinary zooecium the frontal wall of the 

 vestibulum may sometimes be attached to the edge of the operculum, sometimes 

 at a greater or smaller distance within this. This is also the case with the 

 helerozoa»cium, only that the variation is still greater here. While for example 

 the frontal lamina of the vestibulum is attached to the edge of the mandible itself 

 in the small avicularia with a semicircular mandible, which is found in most 

 species of Fhistra, Porella, etc., in the large avicularia of Flnstra abi/ssicola (PI. 

 XIX, fig. 13 a) it is only attached to a triangular median belt, which decreases in 

 breadth distally and does not reach right out to the tip of the mandible and the 

 latter is thus provided with two wing-shaped latei-al parts. In the Onychocella 

 species (PI. XXII, fig. 3d) the frontal lamina is only attached to the proximal part of the 

 mandible over a small triangular area, and still further proximally the attach- 

 ment takes place in the llagelliim of the real vibraculum. As the vestibulum in 

 the heterozooecium as already stated consists of two separate laminae and does not, 

 as in the zooecium, form a funnel-shaped tube, the frontal lamina^ comes into 

 closer relation to the mandible, and for that reason the latter obtains the charac- 

 ter of a two-layered plate, which between its two layers encloses a .space, the 

 mandibular cavity (PI. XIX, 10 b, 13 a, 14 a, 15 a, 15 b), corresponding to the 

 opercular cavity. While the mandible itself is always more or less strongly 

 chitinized and as a rule provided with a rounded spot of thinner nature (the 

 so called »lucida«), near its proximal portion the vestibular covering of the man- 

 dible may sometimes be perfectly membranous, sometimes more or less strongly 

 chitinized over a larger or smaller portion of its surface. It seems thus to be 

 completely membranous in the large avicularia found in a number of Cellepora 

 species, while we very often find in the small avicularia with a semicircular 

 mandible, which so frequently appear in b'liislru, Purella and Cellepora, a dis- 



