ZOOLOGY 



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. 558. ZTymphon hispidum. 1 7, appendages ; id>. abdomen ; . proboscis. 

 (From Laug, after Hoek.) 



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

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

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



internal parasites in certain hydroid 



Zoophytes. 



THE LlNGUATULIDA OR PENTASTOMIDA. 



The Linguatulida (Fig. 559) 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 (hk. ) 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 respiration 

 are wanting. The nervous system is 

 greatly reduced. A narrow nerve-collar 

 surrounds the oesophagus, presenting 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 (Pentastomum 

 t&nioide*) inhabits certain cavities the 

 frontal sinuses and maxillary antra con- 

 nected with the nasal chambers in the 



FIG. 559. Pentastomum taenioides, 



young female, an. anus ; gang, ganglion ; 

 hk. hooks; mo. mouth; ces. oesophagus ; 

 ov. ovary ; ovd. oviduct ; rec. sem. re- 

 ceptaculum seminis ; sex. ap. sexual 

 aperture ; stom. stomach ; ut. uterus. 

 (After Leuckart.) 



