Chap, xiv.] THE TROCHOSPHERE LARVA. 



539 



o 



Bryozoa. It is characterised by the possession of a 

 circlet of long cilia, which separates the anterior 

 portion of the body of the larva (prseoral lobe) from 

 that which lies behind it (Fig. 224) ; this ciliated 

 circlet is retained throughout life by the Rotatoria. 

 In addition to it, other circlets may become developed. 

 The most common of these is that which appears in 

 the region of the anus (telotroclial larvae) ; in 

 others several bands of cilia are formed (polytrochal), 

 and these sometimes encircle the whole body, and are 

 sometimes dorsal and some- 

 times ventral in position. 

 The banded condition is 

 preceded by one in which 

 the cilia are equally distri- 

 buted over the whole body. 



The Trochosphere is 

 provided with a definite di- 

 gestive tract, the lining of 

 which is ciliated ; has a 

 fairly well developed ner- 

 vous system and a sensory 

 apparatus in the prpeoral 

 lobe ; there is also a paired excretory organ, which 

 opens into the body cavity by several funnel-shaped 

 orifices. As the postoral portion increases in length 

 the bands of mesoblastic cells undergo segmentation, 

 and the pra3oral portion becomes proportionately 

 smaller. Later on it develops the tentacles charac- 

 teristic of the Chsetopod, and loses the band, or bands 

 of cilia. 



All the Mollusca have not a free-swimming larva 

 which can be referred to this type ; in the common 

 fresh-water mussel the ova are developed under the 

 shelter of the gills ; here they become provided with a 

 bivalved shell, the free edges of which are toothed ; 

 the larva does not fix itself to its parent by these hooks, 



Fig. 224. Larval Chsetopods. 



o, Mouth ; a, anus ; v, praeoral ; w, post- 

 oral ciliated band. (After Hat- 

 schek.) 



