NEMERTINI 219 



downgrowth of a lobe on either side of the blastopore (Figs. 

 102 C and 103). It now consists therefore of an upper bell- 

 shaped part, which we call the umbrella, and the two pen- 

 dent lateral lobes. Between the two latter, on the under- 

 side of the umbrella, lies the wide mouth-opening (Figs. 102 C 

 and 103). It leads into the oesophagus, which corresponds 

 to an ectodermal invagination, whereas the real entoderm is 

 represented by the intestinal sac back of it (Fig. 102 C). 

 The intestine of the larva, the cells of which are provided 

 with cilia, remains closed. 



Like Turbellarian larvte, the pilidium is encircled by a 

 continuous band of cilia, which fringes the periphery of the 

 umbrella and the margins of the lateral lobes. The ciliation 

 of the band is distinguished from that of the rest of the 

 body by its longer cilia (Figs. 102 G and 103). The par- 

 ticularly stout fiagellum already mentioned takes its origin 

 in a depression at the apex, corresponding to which there is 

 a thickening of the ectoderm. The latter has been compared 

 to the apical plate of the Trocliophore larvae of the Annelida 

 (comp, infra, p. 266). 



As in the annelid Trochophore, two muscle strands, which also seem to 

 contain nerve fibres, issue from the apical plate (Salensky, No. 25). The 

 presence of these cords would not constitute, however, the only resem- 

 blance to the annelid larva, but, according to Salensky, the ciUated band 

 is also accompanied by a nerve cord, which would correspond to the 

 ring-nerve in the ciliated zone of the Trochophore. This nerve cord, which 

 is composed of nerve fibres and ganglionic cells, is said indeed to present 

 a more varied histological difi'erentiation than the ring-nerve of the an- 

 nelid larva. At the point where the nerve cord passes from the lateral 

 lobes to the umbrella, it forms ganglionic swellings, which Salensky 

 interprets as the central organ of the nervous system. 



The inside of the larva, between ectoderm and entoderm, 

 is filled with a gelatinous mass, in which the variously 

 shaped mesenchymatous cells are found embedded (Fig. 102). 

 These become at first the muscle-bands which traverse the 

 larva at regular intervals ; subsequently they become in part 

 the mesodermal elements (connective tissue, musculature, 

 etc.) of the adult animal (Bütschli, No. 2). 



