838 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Pileolaria ; this difference is possibly clue to the difference in the 

 distribution of the nutrient material, for in PsygmohrancJius the deuto- 

 plasm is chiefly collected into the five large cells of the dorsal endo- 

 derm, while, in the form now under examination, it is distributed 

 equally through all the cells of the layer. 



Among the numerous points of interest discussed in the memoir 

 we must here limit ourselves to the nervous system. In the young 

 Pilenlaria it is completely differentiated from the ectoderm, and the 

 cephalic ganglia form two considerable masses in which there is a 

 thick cortical layer and dotted substance ; the commissures have the 

 same structure in the adult as in the embryonic stages, the ventral 

 ganglionic chain is confined to the thoracic portion of the body ; 

 the size of the ventral equals that of the cephalic ganglia, and they 

 have the same structure. This fusion of the ventral chain, though 

 known among Arthropods, is quite an anomalous arrangement among 

 Annelids. 



In the next chapter the history of Aricia fcetida is discussed ; 

 this is a worm which, owing to the large number of its ova and the 

 ease with which they may be cultivated, would be an admirable form 

 for the embryologist, were they not completely opaque, and so 

 spherical that it is difScult to know exactly in what direction the 

 section is being taken. Here, as in the case of some other Annelids, 

 it was noted that among the mass of eggs some did not commence to 

 segment as soon as the rest ; their future history showed, however, 

 that they were normal eggs.* Development takes place by epiboly, 

 and the blastopore is always a large orifice. In its earliest stages the 

 endoderm consists of several large polyhedral cells, with their long 

 axis turned towards the blastopore. On the fourth day the body of 

 the embryo becomes divided into two regions, and the somatic may 

 be distinguished from the cephalic mesoderm ; the origin of the latter 

 was not made out. In Aricia, as in Nereis cultrifera, the eye arises 

 from a single ectodermal cell, which elongates and becomes invaginated; 

 at its proximal end the pigment is collected, and, as in Nereis, the 

 organ is formed before the cephalic ganglia become distinguished as 

 a separate structure. As in all other Annelids the medullary groove 

 is a temporary formation, which disappears a few days after the 

 embryo becomes free, and leaves no traces of its existence ; some of 

 the cells become striated, and on the fifth day of post-embryonic 

 development resemble developing muscle-cells. The ganglionic chain 

 is not developed till very late, the dotted substance only appearing 

 on the fifth day of larval development. In specimens of Aricia six 

 weeks old, the ganglia are still connected with the ectoderm, and the 

 connection between the cephalic and the ventral ganglia is not yet 

 effected. The coelom is also late, though not so late, in appearing ; 

 but the individualization of the metameres in the mesoderm obtains 

 at a much earlier period ; the body-cavity arises independently in 

 each segment, but differs in size in different regions. 



The next chapter deals with Terehella mecJcelii, which has already 



* A similar observation was made by A. P. Tliomas on Distomum hepaticum. 



