ON THE DEVELOPMENT OF LTNGULA ANATINA. 25 



little from the anterior side. In the latter figure we see the 

 tentacle has begun to bend dorsally. Fig. 48 (PI. III.) is 

 another embryo of a similar stage viewed from the anterior side. 

 In this and in Fig. 47, the arm-apparatus arises more markedly 

 from the dorsal mantle lobes. The shell has become so conspicuous 

 that we can detect it without difficulty. 



In consequence of the growth of the tentacle and cirri the 

 arm-apparatus becomes distinctly five-lobed as is seeu in Fig. 49 

 (PI. III.) which represents an embryo from the dorsal side, and 

 in Fig. 50, which is a ventral view of another embryo a little 

 older than that drawn in Fig. 49. The lumen in the arm- 

 apparatus, known as the arm-sinus (am. sn.), can clearly be seen. 

 At the beginning of this stage the part immediately posterior to 

 the mouth is elevated ventrally and unites the two projected 

 ventral ends of the arm-apparatus, so as to form a continuous 

 sheet, so that the mouth is taken into, and placed nearly at the 

 centre of, the arm-apparatus. This process will best be under- 

 stood by comparing Fig. 46 (PI. III.) with Fig. 50 (PL III.). 

 The tentacle is bent dorsally : this is seen especially well when 

 it is projected out of the mantle cavity, but may also be 

 observed in preserved material, as in Fig. 51 (PI. III.) which 

 is drawn from the right side. A comparison of this figure and 

 Fig. 43 will show in what way the embryo has been developed 

 in the interval. The tip of the tentacle at this stage is sometimes 

 a little flattened (PI. III., Fig. 50.). This character is peculiar 

 to the tentacle and is never met with in the cirri. The shape 

 of the mouth (m.) changes according to the degree of contraction ; 

 sometimes it assumes a crescent form, while at others it becomes 

 triangular. A little anterior to the mouth a shelf-like transverse 

 ridge makes its appearance on the arm-apparatus. This repre- 



