THE PYCNOGONIDA. 331 



pair of thoracic limbs of the Spiders, make their appearance 

 beneath the sac of the deutovum. The rudiments of the 

 cheliceras and pedipaipi apply themselves together, and coa- 

 lesce into a proboscis. Thus the first larval form is com- 

 pleted. It tears the pseudoval sac, emerges, and buries 

 itself in the branchiae of the fresh-water mussel ( Unio)^ upon 

 which it is parasitic. The cuticular investment of the first 

 larva now becomes distended by absorption of water, and 

 forms a globular case, the limbs being drawn out of their 

 sheaths. The second larval stage completes itself within the 

 sac formed by this singular ecdysis. The two palpi are de- 

 veloped from the pedipalpal portion of the proboscis; two 

 horny hooks from the cheliceral portion ; and, finally, the 

 hinder pair of thoracic limbs is added. This second larva 

 gradually passes into the adult Atax. 



In the Acarus {3Iyohia coarctata) of the Mouse, Claparede 

 observed that the deutovum stage is followed by a tritovum ; 

 the chitinous sac, which invests the embryo within the deuto- 

 vum, apparently representing the cuticle of the first larva of 

 Atax. In this case, it presents a parallel to the Ncaq?liics 

 cuticle of 3rysis. 



The Arthrogastra^ the Aranema, and the Acarina (with 

 some doubtful exceptions among the latter), possess the same 

 number of appendages, and do not differ from one another so 

 much as do the different forms of the Co]_yepoda,, among the 

 Crustacea. But the remaining groups, which are usually in- 

 cluded among the Arachnida^ namely, the Pycnogonida^ the 

 Arctisca, Sindi the Pentastomida, diverge widely from the Ar- 

 throgastra and the Araneina., though .each exhibits certain 

 approximations to the Acarina. 



The Pycnogonida. — These are marine animals, with short 

 bodies terminated in front by a rostrum like that of the Mites, 

 but with a mere tubercle in place of the posterior thoracic and 

 abdominal somites. The adult has four pairs of enormouslv- 

 elongated, many-jointed, ambulatory limbs, in front of which 

 are three pairs of short appendages, the anterior of which may 

 be chelate, while the posterior are more or less rudimentary 

 (Fig. 91). 



The alimentary canal sends off very long ca:?ca into the 

 legs. There is a short heart, but no distinct respiratory or- 

 gans exist. A cerebral, nervous mass is connected with a 

 ventral chain of four or five pairs of ganglia. Four eyes are 

 seated upon a dorsal tubercle above the brain. The sexes are 



