286 



Arthropoda. 



and in Man Mmself ; if an organ containing such parasites be devoiu-ed by a Dog, 

 they wander into the nasal cavities and complete their development. 



The Fycnogonidse, or Crab- spiders, have a very rudimentary 

 abdomen; the cephalothorax is narrow and divided into four segments. The 

 most anterior is elongated to form a snout-Kke process, at the tip of which is the 

 mouth ; the cephalothoi-ax bears four ocelli, a pair of chelicerse which are usually 

 clawed, and a pair of anteirniform pedipalps, though both these pairs of limbs may 

 be absent ; and also four pairs of eight-jointed legs, which may be thick or 

 very elongate, and which a,lways make up the main mass of the body. In 

 the males, at the base of the first pair of legs, there is a pair of jointed, 

 leg-like appendages, to which the eggs are attached; these appendages 

 may also sometimes ocoui- in the females, which do not carry the eggs. The 



caeca of the alimentary canal extend far into 

 the legs. Respiratory organs ai-e wanting, but 

 a heai-t is present. Thei-e is a pair of ovai-ies, 

 or of testes, which unite posteriorly and send 

 branches into all the legs ; ova and spermatozoa 

 escape by apertures in the second joints of all, or 

 of some of the legs. The newly-hatched lai'vse are 

 unsegmented, and possess only three pairs of 

 limbs, of which the anterior represent small chelse, 

 and are modified to form the chelicerse of the 

 ad\ilt ; the second and third pah-s are short ; the 

 latter apparently degenerate, whUst the second 

 form pedipalpi. Sometimes the larvse are 

 parasitic in Hydrozoa. The Pyenogonidse are marine, crawling slowly about at 

 the bottom of the sea. In noi*them seas both short-legged {Pycnogonum) and 

 long-legged forms {Nymphon) occur. 



The Tardigrada, or Bear -animalcules, are microscopic animals, which 

 live in moss, in gutters, and in fresh water. They are elongate, indistinctly seg- 

 mented, and possess four pairs of stumpy unjointed legs, which have claws at 

 the tips, and by means of which they crawl slowly about. A pair of stylet- 

 shaped stabbing organs may be protruded from the mouth. Respiratory 

 and circulatory organs are absent: on the other hand they possess a pair of 



Fig. 233. Pycnogonum, 



Fig. 234. Diagrammatic figure of a Tardigrade, (? viewed from the left side, a anus, 

 bg ventral ganglion, e cerebral ganglion, d gland, ex excretory organ (P), m stomach, o mouth, 

 P pharynx, t testis. 1 — 4 four legs. — Orig. (with the use of figures by Plate.) 



small eyes and a fairly well- developed nervous system, consisting of a large 

 cerebral ganglion and several distinct ventral ganglia. The sexes are separate; 

 the males are much more rare than the females. If the water, ia which the 

 Tardigrada are living, dries up, they shrink to small granules, and may pass 

 years in this condition ; when they are moistened again they swell out and again 

 become active. The systematic position of this small group is stiU uncertain, and 

 then- location amongst the Arachnida appears to be hardly justified. 



