370 Introdiictiofi to A?ttmal Morphology. 



this arises from the ring are two receptacula seminis. 

 In males the testes also end in a ring vessel. 



In Pycnogonidae the sex-glands have no ducts, 

 but their secretions are emptied into the body cavity, 

 from whence they escape by openings at the bases of 

 all, or one pair of legs. 



Development begins, except in Pseudarachna, by 

 a partial segmentation of the yelk, and the formation 

 of a primitive streak at the ventral side of the embryo ; 

 the young early assume their adult form ; many mites 

 are at first hexapod, until after several moults. The 

 young Pycnogonum has an unjointed body, and two 

 or three pair of two- or three-jointed legs. Pentasto- 

 mum retrogrades in development. The embryo of 

 Ixodes and of scorpions possess the Micropyle appa- 

 ratus of Meissner, indicating their relationship to 

 Archizoea. 



Arachnida are divisible into two sub-classes : — 

 Sub-class I. Pseudarachna {Haeckel) — retrograded forms, 

 with complete yelk-division ; no tracheae, liver, nor renal 

 tubes ; a small epipharyngeal ganglion and a ventral cord of 

 four approximated ganglia. The orders included are dissi- 

 milar. 



Order i . Pantopoda {Burmeisier) — marine or littoral ; in- 

 tegument chitinous ; abdomen rudimental, peg-like, unjointed; 

 cephalothorax four-jointed, bearing four pair of long 7-9 

 jointed, claw-bearing legs ; rarely shorter (Zetes) ; usually as 

 long as, or longer than, the body ; antennary jaws sharp or 

 none (Pycnogonum) ; mandibular palp 4-5 jointed (Nymphon), 

 or none (Pycnogonum) ; ocelli four on a frontal ridge. The 

 narrow oesophagus has three flat, bristled ridges ; the stomach 

 five pair of long glandular caeca, the front, short pair stretch- 

 ing into the antennary jaws ; the testes open at the end of 

 the first or second joint of the divided meropodite, in which 

 the gland lies ; the ovaries lie in the same segment, and open 



