Mr. L. J. Cole on Ten-legged Pycnogonids. 407 



Latreille's second family Pycnoyonides^, of his order Tracii- 

 EAiii^; it possesses all ot" the characters, besides which it 

 has a segment supporting two additional legs, making in all 

 Jive perfect pairs ; this latter circumstance would doubtless 

 bring it in tlie preceding class CRUSTACEA, being a cha- 

 racter which strikingly distinguishes the animals tiuit compose 

 it ; at all events, I think it will certainly form a connecting- 

 link in the great chain of the animal kingdom, between these 

 two classes, passing from the CRUSTACEA into the 

 ARACHNIDES by the genera Nymphon, Phoxichili, 

 Pycnogonum, &c/' 



That the Pycnogonids form a remarkably homogeneous 

 group has often been mentioned, and with the discovery of 

 new forms the existing gaps are rapidly being filled. It is 

 the existence of gaps in our series that enables us to designate 

 species, genera, families, and other taxonomic groups, and as 

 these gaps become gradually filled the demarcation of the 

 groups becomes incieasin^ly difficult. It leads at first either 

 to putting narrower limits upon our various terms and 

 splitting one genus into a number of genera, one species into 

 a number of species, &c., or the formation of new subgroups 

 as new subgenera, subspecies, &c. Both of these processes 

 have been going on extensively in nearly all branches of the 

 animal kingdom ; but as the series becomes more and more 

 complete it becomes more and more difficult to draw lines of 

 distinction — in other words, to find gaps; and as any gap 

 becomes filled our subspecies, species, or genus, or whatever 

 it is, must expand to include all those forms to the next 

 existing break in the series, or else an arbitrary artificial 

 demarcation must be made. In the Pycnogonida the series 

 of characters upon which the present classification depends 

 is so nearly complete — the differences are so small — that the 

 limitations, especially of the larger groups, such as the 

 families, are admittedly arbitrary, and it is a matter upon 

 which hardly two authors agree. But whereas this ])rof usiou 

 of "connecting-links " is a source of much confusion to the 

 taxonomist^ it should enable us to trace very definitely the 

 pliylogcnetic development. Whether this can be done on 

 the basis of the characters at present in use we cannot 

 say until a more careful examination has been made of 

 the anatomical details of other organs, and probably not 

 until the early embryonic and larval development of more 

 of the species is better known, though I believe a thorough 



" * Cuvier, Regne Animal." 



