302 POLYGORDIUS LESS, xxvi 



extremity of the trunk, or what may now be called the anal 

 segment. By this process the larva has assumed the appear- 

 ance of a worm with an immense head and a very slender 

 trunk. 



The original larval stomach (enteron) has extended, with 

 the formation of the metameres, so as to form the greater 

 portion of the intestine : the proctodseum (Prc. dm) is 

 confined to the anal segment. 



Two other obvious changes are the appearance of a pair 

 of small slender processes (A, /) the rudiments of the 

 tentacles on the apex of the prostomium, and of a circlet 

 of cilia (Pr. an. d) round the posterior end of the trunk. 



The internal changes undergone during the assumption of 

 the present form are very striking. In every fully formed 

 metamere the mesoderm solid, it will be remembered, 

 in the previous stage has become divided into two layers, 

 a somatic layer (B and c, Msd (soni) ) in contact with the 

 ectoderm and a splanchnic layer (Msd (spl) ) in contact 

 with the endoderm. The space between the two layers 

 (Cxi] is the permanent body-cavity or ccelome, which is 

 thus quite a different thing from the larval body-cavity 

 or blastoccele, being formed, not as a space between 

 ectoderm and endoderm, but by the splitting of an 

 originally solid mesoderm. 



The division of the mesoderm does not however extend 

 quite to the middle dorsal and middle ventral lines : in both 

 these situations a layer of undivided mesoderm is left (c), 

 and in this way the dorsal and ventral mesenteries are 

 formed. Spaces in these, apparently the remains of the 

 blastocoele, form the dorsal and ventral blood-vessels. More- 

 over the splitting process takes place independently in each 

 segment and a transverse vertical layer of undivided 

 mesoderm (B, Sep) is left separating each segment from the 



