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 

 Biyozoa 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, 6, c). After some time the 

 larva attaches itself and develops the tentacular crown. The primary 

 zooftcium soon produces new zooecia by budding ; avicularia are 



developed, 

 and final- 

 ly, but not 

 until after 

 the death 

 of the old- 



C&-, 



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 

 me.soderm arises from two cells. The larvae 



"SpiSTtSiTS Of the End P^ P an alimentary 



pralia (after Barrois). c, Cypko- 

 nautes (diagrammatic after 

 Hatachek). Oe, mouth; Af, 

 auus; Cb, tuft of cHia; Kn, 

 bud. 



canal bent into the form of a horse-shoe, 

 and a ciliated collar which is protruded 

 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. 



