PHYLUM ARTHROPODA 



625 



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

 to the nauplius form. Additional segments with their appendages are formed 



FIG. 5-20. Nymphon hispidum. 17, appendages ; ab. abdomen ; *, proboscis. 



(From Lang, after Hoek.) 



behind the original three until the form of the adult is completed. Different 

 kinds of Pycnogonids occur at different depths from between tidal limits to 

 considerable depths in the ocean. The larva; of the species of one genus are 

 internal parasites in certain hydroid 

 Zoophytes. 



THE LlNGUATULIDA OR PENTASTOMIDA. 



The Linguatulida (Fig. 521) are para- 

 sitic animals which, when superficially 

 examined, present little appearance of 

 affinity with the Arthropoda. The body 

 is completely worm-like, not divided 

 into regions, and presenting only a super- 

 ficial annulation, which in no way cor- 

 responds with division of the body into 

 segments. The sole representatives of 

 limbs are four hooks (lik.) at the sides 

 of the mouth. The muscular fibres are 

 striated. The alimentary canal is simple 

 and straight, and Malpighian tubes are 

 absent. Heart and organs of respira- 

 tion are wanting. The nervous system 

 is greatly reduced. A narrow nerve - 

 collar surrounds the oesophagus, present- 

 ing no brain enlargement, and connected 

 behind with a single ventral nerve mass. 

 Organs of special sense are absent. 



Some species of Pentastomum are in 

 the adult condition parasites in the 

 lungs of snakes. One species (Pentasto- 

 mum taenioirle*) inhabits certain cavities 

 the frontal sinuses and maxillary antra 

 connected with the nasal chambers, 



VOL. I 



FIG. 521. Pentastomum tsenioides, 



young female, an. anus ; itant/. ganglion ; 

 hi: hooks ; mo. mouth ; its. oesophagus ; 

 or. ovary; ocd. oviduct; rec, .'<. re 

 ceptaculum seminis ; sex. ap. sexual 

 aperture ; stom. stomach ; vt. uterus. 

 (After Leuckart.) 



S S 



