14 THE VOYAGE OF H.M.S. CHALLENGER. 



Before inserting this list I wish to give a short description of the body of a 

 Pycnogonid, and at the same time to state the nomenclature I have made use of. 



The body of every Pycnogonid consists of four segments, the first of which is to be 

 considered as formed by the connection of the head with the first thoracic segment. At 

 the anterior end this first segment is furnished with a long and stout proboscis. This 

 proboscis is situated either about the front of the first segment, as in Nymphon, 

 and in this case is capable of very limited motion, or as in Ammothea and Ascorhynclms, 

 though also situated about the front, it is connected with the segment by means of an 

 articulation, and for that reason is highly movable, or it is, as in Phoxichilidium, 

 situated on the ventral surface of the first segment, and bent forward ; or finally, it is 

 situated about the ventral side, and at the same time lapped over it {BdJimia, mihi). 

 The form and size of this proboscis varies greatly. At its extremity it is furnished with 

 a triangular mouth. It is to be considered as an unpaired outgrowth of the region 

 surrounding the mouth, and has nothing to do wdth a true head, as was supposed by 

 Savigny. Neither is there anatomically or embryologically any real ground for the 

 opinion, suggested by Huxley,^ that the proboscis represents the united chelicerse 

 and pedipalpi like that of Acarina.^ 



The cephalic part of the cephalothoracic segment is generally furnished with three 

 pair of appendages, which long ago received the names of mandibles, palpi, and 

 ovigerous legs. As far as has been ascertained till now, there is not a single genus of 

 Pycnogonid, which does not show these three pair of appendages either in the adult 

 state, or during its embryological development. Yet cases are not rare, in which in 

 the adult animal, either the fu-st (the mandibles) or the second pair (the palpi) or both 

 are deficient. With respect to the third pair of appendages (the so-called ovigerous legs), 

 on the contrary, they are never found wanting, as far as we know, in the adult animal 

 of either sex. Whoever studies different forms of Pycnogouids, will soon discover what 

 a difference may be caused in the appearance of the cephalic part of the body by the 

 presence or absence of the cephalic appendages ; hence it is that the various authors 

 who have proposed a classification of the group have largely made use of this presence 

 or absence of cephalic appendages. Although there is no doubt, I believe, that good 

 characteristics may be derived from the numljer of these appendages, the following may 

 show how extremely necessary it is to be cautious in this matter. 



^ Huxley, Anatomy of Invertebrated Animals, p. 386, London, 1877. 



2 On a tranverse section, the proboscis of the Pycnogoliids always shows a more or less distinctly triangular 

 shape, the mouth is also triangular, &c. The total form, therefore, is to be compared with the fruit of a monocotyle- 

 donous plant, composed of three carpels. Of these one is placed dorsally, the two others meet longitudinally in the 

 middle of the ventral side. If anybody should feel inclined to try again to homologise the proboscis with cephalic ap- 

 pendages, he will have to call the dorsal piece the labrum, and the two others the homologues of mandibles. However, in 

 the earliest stages of development I have observed, the proboscis has ab-eady the form of a short cylindrical appendage, 

 and I must poLat out the anatomical fact that the proboscis for the greater part is innervated from the .'jupraceso- 

 phageal ganglion. 



