COMPAEATIVE MORPHOLOGY OF THE OALEODID.E. 399 



nor for the remains of stigmata found in the Pseudoscorpions and Thelyphonics on segments in the 

 abdomen posterior to the vith *. 



On the other hand, the evidence adduced from the circulatory, respiratory, and alimentary systems 

 of Arachnids (which are in such intimate physiological connection) demands a series which Limulus 

 could not supply, even though the invagination hypothesis had much more to support it than it has. 



23. The Coxal Glands and Maljnghian Vessels. — The primith-e form had excretory 

 glands opening behind the limbs on the 3rd and 5th segments. These may be persistent 

 Annelidan nephridia of these segments. The nephridia have otherwise vanished (? genital 

 dixcts). It is not easy to account for this disappearance of glands of such primary 

 importance to their Annelidan ancestor. 



The single pair of Malpighian vessels openiug into the mid-gut of modern Arachnids 

 are not homologous with the proctodeal Malpighian vessels of the Hexapods, and may 

 perhaps be homologous with one of the numerous pairs of alimentary diverticula 

 of the Ancestral form which became useless for digestive purposes [of. p. 361). Whether 

 these vessels in the Arachnids are really serial with the digesting diverticula in the 

 more anterior segments I cannot say. That such diverticula may become glands is 

 well known, and why not also excretory glands ? 



Excretory glands, assumed to be nephridia (see, however, footnote, p. 380), occur in the cephalothorax 

 in Limuhis, where, owing to the concentration of viscera in this part of the body, they are more 

 explicable than is their limitation to the cephalothorax in the Arachnids. They open on the 5th 

 segment (Tower, Zool. Anz. 1895, p. 471) and are apparently liomologous with the shell-gland oi Apus. 

 Proctodeal Malpighian tubules do not occur in Lhmilus ; in this respect both Limulus and the Arachnids 

 are distinct from the Hexapods. 



24. Spinniiif/-, Stinging-, Cement-, and Stink-glands. — No other group of Arthroj)ods is 

 so abundantly and so variously supplied with such glands as are the Arachnids. As the 

 probable derivatives of setiparous glands of an Annelidan ancestor they suggest 

 that the primitive form was richly supplied with setae. This was indeed the case, as Ave 

 can gather from the great hairiness of the Galeodidse and the Aviculariidoe, and from the 

 immense number of large pores, closely resembling setal pores, through the integument 

 of Scorpio. 



The Merostomata, as specialized for a free swimming life, had generally lost their primitive Annelidan 

 sette. It is very doubtful whether, if they became adapted to a land life, they would be able to 

 redevelop them. This is not all. They would have to be redeveloped in their eai'ly Annelidan form 

 when their secretory glands were still capable of modification into other kinds of glands. For it seems 

 to me that these glands in the Arachnida, occurring in such well-defined series, can only have been 

 derived very early from the series of setiparous areas which occur typically on the parapodia of Chaetopod 

 Annelids (see below, p. 404). 



25. Reproduction. — There is, in the primitive form, a single pair of glands opening 

 together on the first abdominal segment, the limbs of Avhich closed together over the 

 aperture, either to form a sexual organ or a plate-like protection for the aperture. As 



* Jaworowski (35) claims definitely, in his embryological researches, to have proved that the branched tratheiE 

 preceded lung-books, which are but a secondary specializatiou of the former. On the other hand, Wagner (73) 

 maintains that the primitive Arachnid had no tracheae in the cephalothorax ; Galeodes alone proves the contrary 

 inasmuch as its segmentation shows it to be, in this respect at least, a jirimitivc form ((/. also pp. :571-376). 



