334 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



early Arachnidan form, from which the Arthrogastra, Ara- 

 7ieidea, and Acaridea, have branched off. 



The Akctisca, or Tardigrada, are microscopic animals, 

 found, in association with Potifera, in moss and in sand, rare- 

 ly in water, which present many points of resemblance to the 

 Acarina. The body (Fig. 92) is vermiform, with four pairs 

 of tubercles, representing limbs, terminated by two or more 

 claws. The fourth pair is directed backward at the hinder 

 end of the body, so that if these appendages answer to the 

 hinder pair of limbs in the typical Arachnida, the hinder tho- 

 racic, and all the abdominal, somites are undeveloped. The 

 mouth is situated at the extremity of a rostrum provided with 

 two stylets, which is so like that of the Acarina, that it may 

 probably be regarded as formed by the coalescence of cheli- 

 ceral and pedipalpal tubercles. There is a muscular pharynx 

 leading into a wide alimentary canal, which gradually narrows 

 to the anus. No organs of circulation or of respiration exist. 

 The paired ventral ganglia, which correspond in number with 

 the appendages, are large. They are connected by longitu- 

 dinal commissures with one another, and with a prae-cesopha- 

 geal cerebral mass which sometimes bears two eyes. The 

 Arctisca are hermaphrodite, the ovarian sac ana 1 the two 

 testes opening together into a cloacal dilatation in which the 

 intestine terminates. The ova are relatively very large, and 

 the cuticle of the parent is cast off and incloses them when 

 they are laid, as a sort of ephippium. Complete yelk-division 

 takes place. The young have one-third the size of the adult 

 when they are hatched, and they undergo no metamorphosis 

 beyond the addition, in some cases, of one pair of limbs after 

 birth. 1 



The Pextastomida. — A still more aberrant form is the 

 parasitic Linguatula, or Pentastomum, which is found in a 

 sexless condition in the lungs and liver of herbivorous mam- 

 mals and of reptiles, and in the sexual state in the nasal cav- 

 ities and maxillary antra of Carnivores. Thus, as Leuckart's 

 investigations have proved, Pentastomum tcenioides, which 

 inhabits the latter cavities in the dog and the wolf, is the 

 sexual state of the P. denticulatum, which occurs in the liver 

 of hares and rabbits. 2 



1 Kaufmann, " Entwickelung und systematische Stellung der Tardigraden." 

 (Zeit. wiss. Zoolnaie, 1851.) 



2 " Bau und Entwickelungsgeschichte der Pentastonien," 1860. 



