298 



POLYGORDIUS 



LESS. 



ment of other worms, the process, in its general features, 

 is probably as follows : 



The ectoderm and endoderm of the gastrula (Fig. 73, A) 

 are not in close contact with one another as in Fig. 63 

 (p. 265), but are separated by a space filled with fluid the 

 blastocozle or larval body-cavity. The mouth of the gastrula 

 closes (), the enteron (Eni), being thus converted into a 

 shut sac. At about the same time the ectoderm is tucked 



FIG. 73. Diagram illustrating the origin of the trochosphere from 

 the gastrula. The ectoderm is dotted, the endoderm striated. 



A, gastrula, with enteron (Ent} and gastrula-mouth (Cast. Mi/i), and 

 with the ectoderm and endoderm separated by the larval body-cavity or 

 blastocoele (Bl ccel). 



B, the gastrula-mouth has closed, the enteron (Ent} becoming a shut 

 sac. 



c, two ectodermal pouches, the stomodseum (St. dm) and proctodseum 

 (Prc. dm) have appeared. 



D, the stomodaeum (St. dm) and proctodseum (Prc. dm) have opened 

 into the enteron (nt), forming a complete enteric canal with mouth 

 (Mth) and anus (An). 



in or invaginated at two places (C), and the two little 

 pouches (St. dm, Prc. dni) thus formed grow inwards until 

 they meet with the closed enteron and finally open into it 

 (D), so that a complete enteric canal is formed formed, 

 we must not fail to notice, of three distinct parts : (i) an 

 anterior ectodermal pouch, opening externally by the mouth, 

 and distinguished as the stomodceum ; (2) the enteron, lined 

 with endoderm ; and (3) a posterior ectodermal pouch, 

 opening externally by the anus, and called the proctodaum. 



