298 THK DEVELOPMENT 



marked out by a clear space at the anterior end of the mantle 

 and near the forward bend of the intestine. 



If, now, we compare our young Lymneus with the adult land 

 snail, (p. 203, fig. 122,) we shall see, that, notwithstanding the 

 former lacks, as yet, a distinct nervous and reproductive system, 

 there are other parts of the organization which are unmistakably 

 homologous with those of the latter ; and we must conclude, 

 therefore, that at least at this age the embryo conforms to the 

 Molluscan type, whilst we may say, regarding its preceding 

 phases of growth, that there has been nothing seen which did 

 not strictly tend to bring about the result which we have before 

 us. Although it is not necessary for present purposes, I will 

 state that the principal feature which is yet to be developed in 

 order to bring the embryo to the end of its course, is a prolonga- 

 tion of the dorsal region of the body into a long-drawn-out, 

 obliquely conical spiral, closely covered by a similarly shaped 

 shell, within which are included the reproductive organs and the 

 liver. 



ARTICULATA. 



It cannot be repeated too often that the first step which is 

 taken in the development of the young animal, after it has 

 passed through the process of segmentation, is a bipartition of 



the body into a right and a left side. 

 This phenomenon is a very marked 

 feature in the initiatory phase of the 



I 



| ac 2 embryo of the common black Caddice- 

 fly ( Mystacides nig-ra, Latr.) of Europe, 

 a little moth-like insect with very long 

 thread-like feelers and wings slanting 

 like a roof, almost or altogether iden- 

 F 'g- 198. tical with one which flies into our win- 



dows and around the lights in the evenings of early summer. 



Fig. 198. Mystacides nigra. Latr. 220 diam. The earliest stage of devel- 

 opment of the embryo of the black Caddice-fly. c, the cellular envelope ; ac 1 , 

 ac 2 , the edges of c ; y, the yolk-mass not covered by c; d, clearer portion of the 

 yolk mass. From Zaddach. 



