Emhryology of certain of the Lower Fishes. 215 



contractile fibrillre are developed, while the tail-like process 

 is prolonged into the motor nerve fibres. 



The facts which I have mentioned appear to indicate that 

 as regards the Vertebrata the germ layer theory should be 

 stated in some such fashion as this. The vertebrate body 

 consists fundamentally of the two cell-layers — ectoderm and 

 endoderm — each consisting primarily of an epithelial layer 

 of tailed cells. Parts of the primitive endoderm become 

 nipped off to form coelomic lining, including in this, of course, 

 the myocoelic wall, which becomes partially converted into 

 muscle, while individual cells of both primary layers migrate 

 into the cavity between them, and give rise to the mesen- 

 chyme and its derivatives. In the nipped off parts of primi- 

 tive endoderm which line the enterocoelic outgrowths, we 

 may see, with Sedgwick, Gardiner, and others, persisting 

 representatives of the coelenteric pouches of the Scyphozoan 

 Coelenterate and in the immigration of mesenchyme cells a 

 continuation of the similar process so beautifully seen in the 

 immigration of the skeletogenous cells of the Alcyonaria. 



XVI. The Bdelloid Rotifera of the Forth Area. By 

 James Murray. [Plate VII.] 



(Read 27th November 1905.) 



This list of Bdelloid Eotifers from the Forth Area is based 

 entirely on the work of Mr Wm. Evans, who sent to me, in 

 the course of the present year (1905), numerous samples of 

 moss, specially selected for this purpose, gathered on hill-top 

 and in peat-bog, on trees, walls, and rocks, by roadsides and 

 in streams. A few samples of Lemna and other pond weeds, 

 also some seaweed, were likewise sent. This moss, etc., was 

 found to be very rich in microscopic life. A short list of 

 Tardigrada obtained from it has already been published 

 {Ann. Scot. Nat. Hist., 1905, p. 160). Bdelloids were much 

 more numerous ; indeed, among the moss-dwellers no order 

 is so prominent as the Bdelloida. Though they live in 

 other situations, such as the mud of ponds and lakes, and 

 as ecto-parasites on other animals, it is in moss, using the 



VOL. XVI, u 



