18 



BRYOZOA ECTOPROCTA. 



can be distinguished, gives rise to the typical Ectoproctous larva by 

 certain characteristic modifications of the ectoderm. The cells of 

 the corona increase in size and become covered with cilia, giving 

 rise to a transverse ciliated zone (Fig. 7, c), by means of which the 

 body of the larva is divided into an upper aboral half (corresponding 

 to the former animal pole) and a lower oral half. In the aboral 

 half, a disc-like thickening beset at its edge with stiff setae develops 

 (Fig. 7, r) ; this is the so-called retractile or ciliated disc (calotte 

 of Barrois, cap), which has been regarded as the rudiment of 

 the polypide that develops in the primary zooecium of the 

 Bryozoan stock. At its periphery, this organ is encircled by a 

 depression (Fig. 7, p, mantle-cavity Barrois). In the oral half, 

 two organs develop first of all, the plane of symmetry of the larva 

 being marked by their position. In the posterior portion of the 

 oral side a deep ectodermal depression occurs; this originally has 

 the form of an almost completely closed sac (Fig. 7, s), and from 

 the fact that it serves as an attaching organ during the subsequent 

 metamorphosis, it is called the sucker (sac interna, Barrois).* 

 Further forward there is a median furrow beset with strong 

 flagella, the so-called anterior ectodermal furrow (oral furrow 

 of Nitschb, fente of Barrois, dorsal organ of Balfour) ; this is 

 connected internally with a glandular tissue to which Barrois gave 

 the name of pyriform organ (Fig. 7, o). In the space between the 

 anterior ectodermal furrow and the sucker, the oral aperture (Fig. 

 7, m) is found in those larvae in which an alimentary canal develops. 



III. Metamorphosis. 



The metamorphosis of a Bryozoan larva comprises a more or less 

 protracted free-swimming stage, during which no perceptible advance 

 is made in the development of the animal, and further, the attach- 

 ment of the larva, and the subsequent somewhat complicated changes 

 which bring about its transformation into the first primary zooid 

 of the young Bryozoan colony. 



Although there is no difficulty in tracing back the larvae of the 

 Ectoprocta to a common type resembling that indicated above, we 

 find certain distinguishable larval types, which, for the sake of 

 clearness, must be treated separately. The description of each larva 



* It should here he pointed out that the name "sucker" is calculated to 

 convey a false impression as to the function of this ectodermal invagination, 

 which is in reality merely the basal plate of the future primary zooecium in 

 pi. juration and in an invaginated condition. Fixation does not occur through 

 suction, but only after the evagination of this basal plate, as will be seen later. 



