PYCNOGONIDA. 



examinations of the development of the Pycnogonida seem to me to show that in these animals the 

 separation of the nerve mass into ganglia only takes place by degrees, contemporaneously with a 

 corresponding separation of the somites of the trunk, and the separation and growth of the limbs, 

 comp. my figures of the first larval stages in Nymphon macronyx, pi. II, fig. 9, Pscudopallene circiilaris, 

 pi. I, fig. n, and Nymphon Sluiteri, pi. II, fig. 18, and with these figures may be compared the abdom- 

 inal nerve cord in Psrudopallene circularis in the second and [third larval stage, pi. I, fig. 12 and 15. 

 Upon the whole I doubt very much that a coalescing of well separated limbs really takes place, and 

 so much I know at all events from my own examinations that the union, supposed to take place of 

 the arms of the second pair of feet in the females of Lernaeopoda to a fastening knob, is only 

 apparent, each arm in reality ending in an independent knob; the two knobs are only more or less 

 loosely joined to a single knob - - by a somewhat strong pressure they are easily separated. As a 

 paradigm on the coalescing of limbs this pair at all events cannot be used. 



The proboscis is thus, after its origin and structure, especially the want of limbs, not 

 homologous with the other metameres of the body; but even if it be something particular, or, to use 

 the expression of Dohrn organum sui generis*, it can in no way be said of it, as does this author, 

 that im ganzen bisherigen Arthropodentypus nicht seines Gleichen, uicht einmal etwas ihm Aehn- 

 liches* is found, Pantop. Golf. Neap. 1881, p. 13, for I suppose that the proboscis of the Pycnogonida 

 will be found throughout the class of Arachnida, the only difference being that here it is free, large, 

 and predominant, while in the other Arachnida it most frequently is very small and hidden between 

 the gnathites. As an instance of a free proboscis in the Arachnida I shall refer to what in the follow- 

 ing is said of the genus Koenenia. As I have already indicated in the foregoing, in the chapter on 

 the terminology , p. 3 , there is still less reason to interpret it as , or give it the name of, head. A 

 similar structure of the mouth is otherwise found in some mites, as it also in several respects reminds 

 of the mouth in some higher, sucking worms {Hirudo}. In the Insecta, especially in the Tipulida, we 

 find a parallel in the structure of the gullet; and as well in the Pycnogonida as in the Tipulida the 

 production of a pumping or sucking apparatus may be thought to be the conditional reason of the 

 common plan of structure. I think it may be taken to be a consequence of simple, mechanical laws 

 that, when a tube with firm walls is to form a pumping or sucking apparatus, the easiest thing will 

 be to place three plates with contiguous margins longitudinally in the the tube, and by ropes or 

 other suitable means make those plates to approach the wall of the tube at the same time, by which 

 motion the inner opening will be increased, and a pumping or sucking be brought about. With 

 regard to the Tipulae I may refer to my paper, The Gnathites of the Flies. Trophi Dipterorum 

 (1881). On pi. II, fig. 10 of this paper is found a transverse section of the gullet of a Tipulid of the 

 triangular form, so well known from a number of figures and transverse sections of the proboscis of 

 the Pycnogonida. 



For the elucidation of the morphological significance of the proboscis I may also refer to the 

 figures in a recent work by two Danish zoologists, the doctors H. J. Han sen and Will. Sorensen: 

 The Order Palpigradi Thor. (Koenenia mirabilis Grassi) and its Relationship to the other Arachnida* 

 (1898). This Arachnid has already earlier been the subject of a detailed examination and a systema- 

 tical view by Grassi: I Progenitori dei Miriapodi e degli Insetti. Memoria V. Intorno ad un nuovo 



3* 



