THE NEMERTINI 



165 



the body of the Nemertine is formed, and the larval skin is 

 cast off. 



The result of the total and nearly equal segmentation of the 

 egg is a blastula with a capacious blastocoel ; the cells on one side 

 are rather larger than the rest, and by their invagination the 

 larva becomes a gastrula, which is at first uniformly ciliated. 



A special tuft of long sensory hairs is developed in a pit at the 

 aboral pole (Fig. IV.), and by the downgrowth of the sides of the 

 body two or four great lappets are formed ; the organism gradually 

 becomes helmet-shaped (Fig. IV. (7). The margins of these lappets 



FIG. IV. 



Development of the Pilidium larva (after Metschnikoff), from Joubin. A, blastula, already 

 ciliated and moving within the vitelline membrane ; a few mesoblast cells have been given off 

 by the blastoderm ; B, the young gastrula ; C, the larva. The blastocoel is filled with jelly, in 

 which mesoblast cells are embedded ; the side walls have grown down into the right and left 

 lappets ; s, the anterior ; s', the posterior invaginations of the ventral surface which give rise 

 to the "imaginal discs." 



bear long cilia, which are continued round the whole of the ventral 

 edge of the Pilidium, so as to form a circumoral band. Balfour 

 has pointed out the phylogenetic significance of this larva, from 

 which the Trochosphere of Annelids may have been derived. The 

 Pilidium consists of epiblast, of hypoblast, and of mesoblast which 

 gives rise to a jelly-like connective tissue between them. The 

 greater part of the body wall of the future Nemertine is formed 

 out of four invaginations of the ventral surface of the Pilidium 

 a pair of anterior or cephalic, and a pair of posterior or somatic 

 pits (first recognised by Krohn and by Miiller), (Fig. VI.). The 

 wall of each pit soon becomes divisible into two regions the 



