450 TAR DIG RAD A. 



now distinctly differentiated. The remains of two cuticles in the cyst 

 seem to shew that the above changes are effected in two ecdyses. 



In the course of a series of ecdyses the various organs of the larval 

 form known as Pentastonium denticulatum continue to become differentiated. 

 After the first ( = third) ecdysis the resophageal nerve-ring and sexually 

 undifferentiated generative organs are developed. At the fourth ( = sixth) 

 ecdysis the two pairs of hooks of the adult are formed in pockets which 

 appeared at a somewhat earlier stage ; and the body acquires an ammlatecl 

 character. At a somewhat earlier period rudiments of the external gene- 

 rative organs indicate the sex of the larva. 



After a number of further ecdyses, which are completed in about six 

 months after the introduction of the embryos into the intermediate host, 

 the larva attains its full development, and acquires a form in which it has 

 long been known as Pentastonium denticulatum. It now leaves its cyst 

 and begins to move about. It is in a state fit to be introduced into its 

 final host; but if it be not so introduced it may become encysted afresh. 



If the part of a rabbit or hare infected by a Pentastornum denticula- 

 tum be eaten by a dog or wolf, the parasite passes into the nasal cavity of 

 the latter, and after further changes of cuticle becomes a fully-developed 

 sexual Pentastomum ta?nioides, which does not differ to any very marked 

 extent from P. denticulatiim. 



In their general characters the larval migrations of Peutastomum are 

 similar to those of the Cestodes. 



The internal anatomy of the adult Pentastomum, as well as the cha- 

 racters of the larva with two pairs of clawed appendages, are perhaps 

 sufficient to warrant us in placing it with the Arthropoda, though it 

 would be difficult to shew that it ought not to be placed with such 

 a form as Myzostomum (vide p. 305). There do not appear to be any 

 sufficient grounds to justify its being placed with the Mites amongst the 

 Arachnida. If indeed the rings of the body of the Pentastomida are to 

 be taken as implying a true segmentation, it is clear that the Pentastomida 

 cannot be associated with the Mites. 



BIBLIOGRAPHY. 



(539) P. J. van Beneden. "Eecherches s. Torganisation et le d^veloppement 

 d. Linguatnles." Ann. d. Scien. Nat., 3 Ser., Vol. xi. 



(540) E. Leuckart. "Bau u. Entwicklimgsgeschichte d. Pentastomen." Leipzig 

 and Heidelberg. 1860. 



TARDIGRADA. 



Very little is known with reference to the development of the Tardi- 

 grada. A complete and regular segmentation (von Siebold, Kaufmann, No. 

 541) is followed by the appearance of a groove on the ventral side indicat- 

 ing a ventral flexure. At about the time of the appearance of the groove 

 the cells become divided into an epiblastic investing layer and a central 

 hypoblastic mass. 



The armature of the pharynx is formed very early at the anterior 

 extremity, and the limbs arise in succession from before backwards. 



The above imperfect details throw no light on the systematic position 

 of this group. 



Tardigrada. 



(541) J. Kaufmann. "Ueber die Eutwicklung u. systematische Stelluug d. 

 Tardigraden." Zeit. f. wiss. Zool., Bd. HI. 1851. 



