152 



PANTOPODA. 



rods arise through fusion of the rods of two neighbouring cells, makes the 

 similarity appear still more striking, and leads to the same conclusion in both 

 cases ; viz., to a derivation of these apparently simple eyes from compound eyes. 

 Our knowledge of the eyes of the Pantopoda is, however, still too slight to 

 allow of any definite conclusions ; Morgan even adopts an altogether opposite 

 view, and explains the inversion which in all cases is present in these eyes, by 

 the degeneration of the posterior part of an optic invagination and the better 

 development of the anterior part. In this way he derives the inverted 

 Pantopodan eyes from such simple eyes (ocelli) as those of the Insecta, being 

 guided in this decision chiefly by a certain bilateral symmetry in the Panto- 

 podan eye. But that method of development as it appears in the ontogeny of 

 the eye, i.e., the growth of the invagination towards one side, is merely a 

 caenogenetic process, and serves for the quicker attainment of the structure now 

 possessed by the adult eye. It has the significance of an abbreviated develop- 

 ment. As a logical consequence of this view, a corresponding assumption must 

 be made for the Arachnid eyes. We cannot here examine Morgan's conclusions 



Fig. 71. — Larvae of Tanystylum in two different stages, seen from the ventral side (after 

 Morgan), a, anus ; aid, abdomen ; bg, ventral chain of ganglia ; m, mouth ; s, proboscis ; 

 I-IV, first four limbs. 



more closely, but refer to the original treatise and to our own view of the eyes 

 of the Arachnida given above (p. 68). On the other hand, it must be mentioned 

 that the description recently given by Clatjs (Xo. 2) of the origin of the 

 median eye in the Crustacea, involuntarily recalls the condition of the eyes 

 in the Pantopoda. The median eyes of the Crustacea are said by Clat/s to 

 arise by inversion, and seem to have their elements arranged like those of the 

 Pantopodan eyes. The rods lie on the inner side, directed towards the pigment- 

 cup of the eye. while the nerve-fibres join them from the opposite side, where 

 also lie the nuclei of the retinal cells. 



The principal change which brings about the transformation of 

 the larva into the adult is the formation of new segments at the 

 posterior part of the body. The limbs already present either pass 

 over directly to the adult, merely growing and developing further, or 



