V PHYLUM PLATYHELBIINTHES 275 



become formed is, ns in the Ctenophora (p. 218), a process of 

 cpibolic gasfrukition. The brain is developed from a pair of 

 thickenings of the ectoderm : these unite into a common mass from 

 which the longitudinal nerves are formed as backward outgrowths. 

 The mouth is developed as an ingrowth from the ectoderm in the 

 position of the former blastopore, the involuted epidermal cells 

 giving rise to the epithelium of the pharnyx and pharyngeal sac, 

 while the muscular tissues of the wall of the pharnyx are formed 

 from surrounding mesodermal elements. The intestine is at 

 first simple in form ; the caeca are developed as a result of 

 the formation of vertical mesodermal septa which, growing inwards, 

 constrict the enteric wall and the enclosed mass of nutrient 

 material. The embryo, which has assumed an ellipsoidal shape, 

 becomes flattened in the dorso-ventral direction, and, having 

 absorbed the greater part of the nutrient matter, escapes by 

 rupture of the egg-shell. 



In many cases the embryo develops into a characteristic larval 

 form, such as that known as Midler's larva (Fig. 217). It assumes 



Fig. 217.— Miiller's larva. A, longitudinal section ; B, lateral view. cc. ectoderm ; en. 

 endodemi ; g. biiiiii; hd. enterori ; o. mouth; ph, pharynx; 'fit. pharyngeal pouch; sn 

 Slicker. (From Lang.) 



an oval shape, with a series of eight elongated processes, covered 

 with long cilia, and connected together by a ciliated band. There 

 are eye-spots at the anterior end and a mouth in the middle of 

 the ventral surface. The form of the body alters after a time, 

 becoming gradually longer and flatter, and the arms are 

 gradually reduced in length, till, eventually, they become completely 

 absorbed. 



The development of the Triclads is very difierent from that of 

 the Polyclads. Each egg-capsule or cocoon encloses a number of 

 oosperms and a quantity of yolk-cells. After segmentation a 

 blastoderm is formed composed of a rounded mass of cells 

 surrounded by the yolk-material, the cells of which become more 

 or less fused. Around the periphery of the blastoderm a layer 

 of cells form a thin membrane — the ectoderm. About the 

 middle appears a rounded group of cells which passes to the 

 periphery and becomes connected with the ectoderm ; a cavity 

 is formed in its interior and it becomes converted into the 



T 2 



