218 HYATT, 



elusions in Cristatella. The rupture of the sheath and the 

 consequent escape of its contents is not an uncommon oc- 

 currence among the fixed statoblasts of Plumatella; and 

 this seems to have been the cause of the emptiness of the 

 specimens described by Prof. Allman. From their mode 

 of development, and the place they occupy in the ccenoeci- 

 um, it is probable that they are the same as the fixed sta- 

 toblasts of Plumateila. They differ, however, from the 

 fixed statoblasts in being unattached to the endocyst when 

 fully grown, but this not being an invariable character, 

 and the elliptical depression, which is nothing more than 

 the accidental sinking in of one side of the sheath, being 

 quite common, even among the free statoblasts of Pluma- 

 tella, I see no reason for considering them exceptional 

 forms. 



At an early stage of growth, while still floating freely in 

 its native element, the statoblastic polypide begins to mul- 

 tiply by the process of budding. An internal swelling of 

 the endocyst, on the lower side, in the vicinity of the bases 

 of the anterior retentor muscles, first shows the position of 

 the coming polypide. This elongates into a little hollow 

 sack with a thickened rim (PI. 7, fig. 5, Y), upon the up- 

 per edge of which, in the Hypocrepian Polyzoa, a slight 

 notch is formed by the duplication and pushing out of its 

 sides into two loops joined along the centre (PL 13, fig. 

 4, Y). A series of minute folds of the membrane on the 

 upper sides of the loops are the incipient tentacles, and, as 

 they enlarge, the intervening membrane is drawn up with 

 them like a thick web ; but this, however, eventually recedes 

 externally and becomes the calyx. The loops growing out- 

 ward augment their longitudinal diameter at the expense 

 of the transverse, and the inner sides of each, approximat- 

 ing and at last coalescing, make up the lophophore and 

 arms. Preceding the beginning of the tentacles, a trans- 

 verse constriction of the body of the little sack draws the 

 line between the oesophagus, and the stomach ; and the 

 subsequent deepening of this constriction divides off the 

 internal cavity, establishing the cardiac and pyloric valves. 

 The muscles, which become well differentiated at a very 

 early period, are divisible into three pairs : one pair attach- 

 ed to the rim, the Brachial Retractors; one to the region 



