76 



MOLLUSCOIDEA. 



so that the ciliated embryo still enclosed in the egg membranes repre- 

 sents a small colony of two individuals. In the marine chilostomatous 

 Bryozoa the fertilized egg passes into the ovicell, which consists of 

 a helmet-shaped capsule and a vesicular operculum. Here the egg 

 segments and develops into an embryo, which passes out as a ciliated 

 larva, and swims about freely in the sea. The irregularly globular 

 larva possesses a ring of cilia (fig. 550, a, b, c). After some time the 

 larva attaches itself and develops the tentacular crown. The primary 

 zocRcium soon produces new zocecia by budding; avicularia are 

 a develop ed,. 



and final- 

 ly, but not 

 until after 

 the death' 

 of the old- 

 er zooecia, 

 root fila- 

 ments. 



In the Endoprocta the egg develops in 

 a brood-pouch placed on the oral side of 

 the animal. The segmentation is complete, 

 and leads to the formation of a blasto- 

 sphere ; the endoderm arises by invagina- 

 tion, and gives rise to the lining of the 

 midgut ; the oesophagus and rectum being 

 formed from the ectoderm (fig. 551). The 

 mesoderm arises from two cells. The larvae 

 of the Endoprocta possess an alimentary 

 canal bent into the form of a horse-shoe, 

 and a ciliated collar which is protruded 



C6 



Cb 



FiG.550. a, Larva of Candareptans 

 (after Barrois). b, Larva of Le- 

 pralia (after Barrois). c, Cypho- 

 nautes (diagrammatic after 

 Hatschek). Oe, mouth ; Af, 



auus ; Cb, tuft of cilia ; Kn, at the front end ; further, they contain a 

 bud (fig. 551 e, Kn), as the first rudiment 

 of a second individual, and a cement gland at the hind end (Dr). 



Other larval forms, which are apparently of a very different 

 structure, are reducible to the same type e.g., Cyphonautes 

 (fig. 500, c), a larva which is found in all seas, and is, according to 

 Schneider, the larva of Membranipora pilosa. 



After the winter the contents of the statoblasts give rise to simple, 

 non-ciliated animals, which possess, when they are hatched, all the 

 parts of the adult animal, at once become attached, and produce 

 new colonies by budding. 



