PHYLUM ARTHROPODA 



657 



APPENDIX TO THE ARACHNIDA. 



THE PYCNOGONIDA, LINGtTATTJLIDA, AND TARDIGRADA. 



These three groups, though not in any way related to one another, and of 

 doubtful relationships to the Arachnida, are, as a matter of convenience, 

 mentioned together here. 



THE PYCNOGONIDA. 



These are marine Spider-like Arthropods (Fig. 568) in which the body 

 consists of a cephalothorax composed of an anterior proboscis (s), three head- 

 segments, and one thoracic segment, followed by three free thoracic segments 

 and a rudimentary abdomen (ab.). The cephalothorax bears usually four 

 simple eyes and four pairs of appendages, the first of which may be chelate. 

 To these succeed a pair of ovigerous legs (3), and the first pair of thoracic 

 legs (4). The free thoracic segments bear lateral processes for the articulation 



Fi. 508. Nymphon hispidum. 1 7, appendages ; ab. abdomen ; s. proboscis. 



(From Lang, after Hoek.) 



of the remaining three pairs of legs. A number of Antarctic species have 

 five pairs of legs. The rudimentary abdomen (ab.) is devoid of appendages. 



Diverticula from the mesenteron penetrate for a considerable distance into 

 the limbs. Malpighian vessels are absent. There is a tubular heart with two 

 or three pairs of ostia. Organs of respiration are absent. The nervous 

 system consists of brain, sub-oasophageal ganglia and three other ganglia in 

 the cephalothorax, and one or two small pairs in the abdomen. The testes in 

 the male are partly, and the ovaries in the female either partly or completely, 

 contained in the bases of the thoracic appendages on which they open. In the 

 male 4-7 cement-glands are situated in the fourth joints of certain of the 

 appendages ; their secretion cements the eggs together into masses which are 

 carried on the ovigerous legs of the male, and in one species on those of the 

 female also. 



A metamorphosis occurs in most cases. The larva usually has three pairs 

 oi^appendages, so that it bears a superficial resemblance to a nauplius ; but 

 the appendages are simple, and in other respects the larva has no essential 

 likeness to the nauplius form. Additional segments with their appendages 

 are formed behind the original three until the form of the adult is completed. 



